​MN 93 Assalayana Sutta: To Assalayana 

Thus have i heard.

On one occasion the Blessed One was living at Savatthi in Jeta's Grove, Anathapindika's Park. Now at that time 500 brahmins from diverse provinces were staying at Savatthi for some business or other matters. Then those brahmins thought: "This recluse Gotama describes purification for all the four castes. Who is there able to dispute with him about this assertion?"

Now on that occasion a brahmin student named Assalayana was staying at Savatthi. Young, shaven-headed, sixteen years old, he was a master of the Three Vedas with their vocabularies, liturgy, phonology, and etymology, and the histories as a fifth; skilled in philology and grammar, he was fully versed in natural philosophy and in the marks of a Great Man. Then the brahmins thought: "There is this young brahmin student named Assalayana staying at Savatthi. Young.. .fully versed in natural philosophy and in the marks of a Great Man. He will be able to dispute with the recluse Gotama about this assertion." So the brahmins went to the brahmin student Assalayana and said to him: "Master Assalayana, this recluse Gotama describes purification for all the four castes. Let Master Assalayana come and dispute with the recluse Gotama about this assertion."

When this was said, the brahmin student Assalayana replied: "Sirs, the recluse Gotama is one who speaks the Dhamma. Now those who speak the Dhamma are difficult to dispute with. I am not able to dispute with the recluse Gotama about this assertion." A second time the brahmins said to him: "Master Assalayana, this recluse Gotama describes purification for all the four castes. Let Master Assalayana come and dispute with the recluse Gotama about this assertion. For the training of a wanderer has been completed by Master Assalayana."
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For the second time the brahmin student Assalayana replied: "Sirs, the recluse Gotama is one who speaks the Dhamma. Now those who speak the Dhamma are difficult to dispute with. I am not able to dispute with the recluse Gotama about this assertion." A third time the brahmins said to him: "Master Assalayana, this recluse Gotama .... Let Master Assalayana come and dispute with the recluse Gotama about this assertion. For the training of a wanderer has been completed by Master Assalayana. Let not the Master Assalayana be defeated without having even fought the battle." 
When this was said, the brahmin student Assalayana replied: "Sirs, the recluse Gotama is one who speaks the Dhamma. Now those who speak the Dhamma are difficult to dispute with. I am not able to dispute with the recluse Gotama about this assertion. Still, sirs, at your bidding I will go."

Then the brahmin student Assalayana went with a large number of brahmins to the Blessed One and exchanged greetings with him. When this courteous and amiable talk was finished, he sat down at one side and said to the Blessed One: "Master Gotama, the brahmins say thus: 'Brahmins are the highest caste, those of any other caste are inferior; brahmins are the fairest caste, those of any other caste are dark; only brahmins are purified, not non-brahmins; brahmins alone are the sons of Brahma, the offspring of Brahma, born of his mouth, born of Brahma, created by Brahma, heirs of Brahma.' What does Master Gotama say about that?"

Buddha: "Now, Assalayana, the brahmin women are seen having their periods, becoming pregnant, giving birth, and lactation. And yet those who are born from the wombs of the brahmin women say thus: 'Brahmins are the highest caste...brahmins alone are the sons of Brahma, the offspring of Brahma, born of his mouth, born of Brahma, created by Brahma, heirs of Brahma.'"

Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste..... .heirs of Brahma.'"
Buddha: "What do you think, Assalayana? Have you heard that in Yona and Kamboja and in other out land countries there are only two castes, masters and slaves, and that masters become slaves and slaves masters?"
Assalayana: "So I have heard, sir."
Buddha: ​"Then on the strength of what argument or with the support of what authority do the brahmins in this case say thus: 'Brahmins are the highest caste.. .heirs of Brahma'?"
Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste.. .heirs of Brahma.'"

Buddha: "What do you think, Assalayana?Suppose a noble were to kill living beings, take what is not given, misconduct himself in sensual pleasures, speak falsely, speak maliciously, speak harshly, gossip, be covetous, have a mind of ill will, and hold wrong view. On the dissolution of the body, after death, would he likely to reappear in a state of deprivation, in an unhappy destination, in perdition, even in hell - and not a brahmin? Suppose a merchant.. .....a worker were to kill living beings.. .and hold wrong view. On the dissolution of the body after death, would he likely reappear in a state of deprivation, in an unhappy destination, in perdition, even in hell - and not a brahmin?"

Assalayana: "No, Master Gotama. Whether it be a noble, or a brahmin, or a merchant, or a worker - those of all four castes who kill living beings...and hold wrong view, on the dissolution of the body, after death, they are likley to reappear in a state of deprivation, in an unhappy destination, in perdition, even in hell."
Buddha: "Then on the strength of what [argument] or with the support of what [authority] do the brahmins in this case say thus: 'Brahmins are the highest caste.. .heirs of Brahma'?"

Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste.. .heirs of Brahma.'"
Buddha: "What do you think, Assalayana? Suppose a brahmin were to abstain from killing living beings, from taking what is not given, from misconduct in sensual pleasures, from false speech, from malicious speech, from harsh speech, and from gossip, and were to be uncovetous, to have a mind without ill will, and to hold right view. On the dissolution of the body, after death, would only he likely reappear in a happy destination, even in the heavenly world - and not a noble, or a merchant, or a worker?"
Buddha: "No, Master Gotama. Whether it be a noble, or a brahmin, or a merchant, or a worker - those of all four castes who abstain from killing living beings...and hold right view, on the dissolution of the body, after death, are likely to reappear in a happy destination, even in the heavenly world." 
Buddha: "Then on the strength of what argument or with the support of what authority do the brahmins in this case say thus: 'Brahmins are the highest caste.. .heirs of Brahma'?"

Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste.......heirs of Brahma.'"
Buddha: "What do you think, Assalayana? Is only a brahmin capable of developing a mind of loving-kindness towards a certain region, without hostility and without ill will, and not a noble, or a merchant, or a worker?"
Assalayana: "No, Master Gotama. Whether it be a noble, or a brahmin, or a merchant, or a worker - those of all four castes are capable of developing a mind of loving-kindness towards a certain region, without hostility and without ill will."

Buddha: "Then on the strength of what argument or with the support of what authority do the brahmins in this case say thus: 'Brahmins are the highest caste.. .heirs of Brahma'?"
Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste.. .heirs of Brahma.'"

Buddha: "What do you think, Assalayana? Is only a brahmin capable of taking a loofah and bath powder, going to the river, and washing off dust and dirt, and not a noble, or a merchant, or a worker?"
Assalayana: "No, Master Gotama. .Whether it be a noble, or a brahmin, or a merchant, or a worker - those of all four castes are capable of taking a loofah and bath powder, going to the river, and washing off dust and dirt."

Buddha: "Then on the strength of what argument or with the support of what authority do the brahmins in this case say thus: 'Brahmins are the highest caste.. .heirs of Brahma'?"
Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste.. .heirs of Brahma.'"
Buddha: "What do you think, Assalayana? Suppose a noble king were to assemble here a 100 men of different birth and say to them: 'Come, sirs, let any here who have been born into a noble clan or a brahmin clan or a royal clan take an upper fire-stick of sala wood, salala wood, sandalwood, or padumaka wood and light a fire and produce heat. And also let any who have been born into an outcast clan, a trapper clan, a wicker workers' clan, a cartwrights' clan, or a scavengers' clan take an upper fire-stick made from a dog's drinking trough, from a pig's trough, from a dustbin, or from castor-oil wood and light a fire and produce heat.' "
"What do you think, Assalayana? When a fire is lit and heat is produced by someone in the first group, would that fire have a flame, a color, and a radiance, and would it be possible to use it for the purposes of fire, while when a fire is lit and heat is produced by someone of the second group, that fire would have no flame, no color, and no radiance, and it would not be possible to use it for the purposes of fire?"

Assalayana: "No, Master Gotama. When a fire is lit and heat is produced by someone in the first group, that fire would have a flame, a color, and a radiance, and it would be possible to use it for the purposes of fire. And when a fire is lit and heat is produced by someone of the second group, that fire too would have a flame, a color, and a radiance, and it would be possible to use it for the purposes of fire. For all fire has a flame, a color, and a radiance, and it is possible to use all fire for the purposes of fire."

Buddha: "Then on the strength of what argument or with the support of what authority do the brahmins in this case say thus: 'Brahmins are the highest caste.. .heirs of Brahma'?"
Assalayana: "Although Master Gotama says this, still the brahmins think thus: 'Brahmins are the highest caste.. .heirs of Brahma.'"
Buddha: "What do you think, Assalayana? Suppose a noble youth were to cohabit with a brahmin girl, and a son was born from their cohabitation. Should a son born from a noble youth and a brahmin girl be called a noble after the father or a brahmin after the mother?"
Assalayana: "He could be called both, Master Gotama."

Buddha:"What do you think, Assalayana? Suppose a brahmin youth here were to cohabit with a noble girl, and a son were to be born from their cohabitation. Should the son born from a brahmin youth and a noble girl be called a noble after the mother or a brahmin after the father?"
Assalayana: "He could be called both, Master Gotama."
Buddha: "What do you think, Assalayana? Suppose a mare were to be mated with a male donkey, and a foal were to be born as the result. Should the foal be called a horse after the mother or a donkey after the father?" 
Assalayana: "It is a mule, Master Gotama, since it does not belong to either kind. I see the difference in this last case, but I see no difference in either of the former cases." 
Buddha: "What do you think, Assalayana? Suppose there were two brahmin students who were brothers, born of the same mother, one studious and acute, and one neither studious nor acute. Which of them would brahmins feed first at a funeral feast, or at a ceremonial milk-rice offering, or at a sacrificial feast, or at a feast for guests?"
Assalayana: "On such occasions, brahmins would feed first the one who was studious and acute, Master Gotama; for how could what is given to one who is neither studious nor acute bring great fruit?"

Buddha: "What do you think, Assalayana? Suppose there were two brahmin students who were brothers, born of the same mother, one studious and acute, but immoral and of bad character, and one neither studious nor acute, but virtuous and of good character. Which of them would brahmins feed first at a funeral feast, or at a ceremonial milk-rice offering, or at a sacrificial feast, or at a feast for guests?"
Assalayana: "On such occasions, brahmins would feed first the one who was neither studious nor acute, but virtuous and of good character, Master Gotama; for how could what is given to one who is immoral and of bad character bring great fruit?"

Buddha: "First, Assalayana, you took your stand on birth, and after that you took your stand on scriptural learning, and after that you have come to take your stand on the very ground that purification is for all four castes, as I describe it." When this was said, the brahmin student Assalayana sat silent and dismayed, with shoulders drooping and head down, glum, and without response.

Knowing this, the Blessed One said to him:"Once, Assalayana, when seven brahmin seers were dwelling in leaf huts in the forest, this pernicious view arose in them: 'Brahmins are the highest caste.....heirs of Brahma.' Now the seer Devala the Dark heard this. Then he arranged his hair and beard, dressed in ochre-coloured garments, put on stout sandals, and taking a staff made of gold, he appeared in the courtyard of the seven brahmin seers. Then, while walking up and down the courtyard of the seven brahmin seers, the seer Devala the Dark spoke thus: 'Where have those worthy brahmin seers gone? ' Then the seven brahmin seers thought: 'Who is walking up and down in the courtyard of the seven brahmin seers like a village lout speaking thus: "Where have those worthy brahmin seers gone? " Let us curse him!' Then the seven brahmin seers cursed the seer Devala the Dark thus: 'Be ashes, vile one! Be ashes, vile one!' But the more the seven brahmin seers cursed him, the more comely, beautiful, and handsome the seer Devala the Dark became. Then the seven brahmin seers thought: 'Our asceticism is in vain, our holy life is fruitless; for formerly when we cursed anyone thus: "Be ashes, vile one! Be ashes, vile one!" he always became ashes; but the more we curse this one, the more comely, beautiful, and handsome he becomes.'
'"Your asceticism is not in vain, sirs, your holy life is not fruitless. But, sirs, put away your hatred towards me.'
'"We have put away our hatred towards you, sir. Who are you?' '
"Have you heard of the seer Devala the Dark, sirs?'
'Yes, sir.'
'I am he, sirs.'
"Then the seven brahmin seers went to the seer Devala the Dark and paid homage to him. Then he said to them: 'Sirs, I heard that while the seven brahmin seers were dwelling in leaf huts in the forest, this pernicious view arose in them: "Brahmins are the highest caste.. .heirs of Brahma.'"
'That is so, sir.' '

"But, sirs, do you know if the mother who bore you went only with a brahmin and never with a non-brahmin?'
'No, sir.' '
"But, sirs, do you know if your mother's mothers back to the seventh generation went only with brahmins and never with non-brahmins?'
'No, sir.' '
"But, sirs, do you know if the father who begot you went only with a brahmin woman and never with a non-brahmin woman?'
'No, sir.' '
"But, sirs, do you know if your father's fathers back to the seventh generation went only with brahmin women and never with non-brahmin women?'
'No, sir.' '
"But, sirs, do you know how the conception of an embryo in a womb comes about?' '
"Sir, we know how the conception of an embryo in a womb comes about. Here, there is a union of the mother and father, and it is the mother's season, and the being to be reborn is present. Thus the conception of an embryo in a womb comes about through the union of these three things.'

"Then, sirs, do you know for sure whether that being to be reborn is a noble, or a brahmin, or a merchant, or a worker?' '
"Sir, we do not know for sure whether that being to be reborn is a noble, or a brahmin, or a merchant, or a worker.' "'That being so, sirs, then what are you?' '
"That being so, sir, we do not know what we are.'

"Now, Assalayana, even those seven brahmin seers, on being pressed and questioned and cross-questioned by the seer Devala the Dark on their own assertion about birth, were unable to support it. But how shall you, on being pressed and questioned and cross-questioned by me now on your assertion about birth, be able to support it? You, who rely on the teachers' doctrines, are not even fit to be their spoon-holder Punna."

When this was said, the brahmin student Assalayana said to the Blessed One: "Magnificent, Master Gotama! Magnificent, Master Gotama!...(as Sutta 91)...From today let Master Gotama remember me as a lay follower who has gone to him for refuge for life."  
In summary, a group of brahmins asked a young brahmin Assalayana to go to the Buddha to debate that brahmins are the highest caste . The brahmin student Assalayana replied: "Sirs, the recluse Gotama is one who speaks the Dhamma. Now those who speak the Dhamma are difficult to dispute with. I am not able to dispute with the recluse Gotama about this assertion." After multiple rejections to go, the brahmins persisted in encouraging Assalayana to go challenge the Buddha.

Finally Assalayana went to the Buddha and  said to the Blessed One: "Master Gotama, the brahmins say thus: 'Brahmins are the highest caste, those of any other caste are inferior; brahmins are the fairest caste, those of any other caste are dark; only brahmins are purified, not non-brahmins; brahmins alone are the sons of Brahma, the offspring of Brahma, born of his mouth, born of Brahma, created by Brahma, heirs of Brahma.' What does Master Gotama say about that?" 

Buddha firstly use science to rebut the brahmin's claim that "brahmin is born from Brahma's mouth; he said all human beings are born from the wombs of their respective mothers. This is followed by Buddha's thoroughly rational and convincing set of arguments for the position that it is individual merit, not birth that distinguishes one man from another; thus all castes are equal. 

Finally Assalayana was convinced by the Buddha that his theory of "Brahmins are the highet caste" was wrong. He became a lay follower of the Buddha and took refuge in him. 
References: 
1. www.accesstoinsight.org
2. https://suttacentral.net/
3. The middle length discourses of the Buddha (Bhikkhu Bodhi)