MN 58 Abhayarajakumara Sutta: To Prince Abhaya 

Thus have i heard. 

On one occasion the Blessed One was living at Rajagaha in the Bamboo Grove, the Squirrels' Sanctuary. Then Prince Abhaya went to the Nigantha Nataputta, and after paying homage to him, sat down at one side. Thereupon the Nigantha Nataputta said to him: "Come, prince, refute the recluse Gotama's doctrine, and a good report of you will be spread to this effect: 'Prince Abhaya has refuted the doctrine of the recluse Gotama, who is so powerful and mighty.'"
Prince Abhaya: "But how, venerable sir, shall I refute his doctrine?"
Nigantha Nataputta; "Come, prince, go to the recluse Gotama and say: 'Venerable sir, would the Tathagata utter speech that would be unwelcome and disagreeable to others?' If the recluse Gotama, on being asked thus, answers: 'The Tathagata would utter speech that would be unwelcome and disagreeable to others,' then say to him: "Then, venerable sir, what is the difference between you and an ordinary person? For an ordinary person also would utter speech that would be unwelcome and disagreeable to others.' But if the recluse Gotama, on being asked thus, answers: "The Tathagata would not utter speech that would be unwelcome and disagreeable to others,' then say to him: 'Then, venerable sir, why have you declared of Devadatta: "Devadatta is destined for the states of deprivation, Devadatta is destined for hell, Devadatta will remain in hell for the aeon, Devadatta is incorrigible"? Devadatta was angry and dissatisfied with that speech of yours.' When the recluse Gotama is posed this two-horned question by you, he will not be able either to gulp it down or to throw it up. If an iron spike were stuck in a man's throat, he would not be able either to gulp it down or to throw it up; so too, prince, when the recluse Gotama is posed this two-horned question by you, he will not be able either to gulp it down or to throw it up." 

 "Yes, venerable sir," Prince Abhaya replied. Then he rose from his seat, and after paying homage to the Nigantha Nataputta, keeping him on his right, he left and went to the Blessed One. After paying homage to the Blessed One, he sat down at one side, looked at the sun, and thought: "It is too late today to refute the Blessed One's doctrine. I shall refute the Blessed One's doctrine in my own house tomorrow." Then he said to the Blessed One: "Venerable sir, let the Blessed One with three others consent to accept tomorrow's meal from me." The Blessed One consented in silence. Then, knowing that the Blessed One had consented, Prince Abhaya rose from his seat, and after paying homage to him, keeping him on his right, he departed.

The next morning, the Blessed One dressed, and taking his bowl and outer robe, he went to Prince Abhaya's house and sat down on the seat made ready. Then, with his own hands, Prince Abhaya served and satisfied the Blessed One with various kinds of good food. When the Blessed One had eaten and had withdrawn his hand from the bowl, Prince Abhaya took a low seat, sat down at one side, and said to the Blessed One:  "Venerable sir, would a Tathagata utter such speech as would be unwelcome and disagreeable to others?"
Buddha: "There is no one-sided answer to that, prince."
Prince : "Then, venerable sir, the Niganthas have lost in this."
Buddha: "Why do you say this?"

Prince Abhaya then reported to the Blessed One his entire conversation with the Nigantha Nataputta. At that time, a young tender infant was lying prone on Prince Abhaya's lap. Then the Blessed One said to Prince Abhaya: "What do you think, prince? If, while you or your nurse were not attending to this child and if he were to put a stick or a pebble in his mouth, what would you do to him?"
Prince: "Venerable sir, I would take it out. If I could not take it out at once, I would take his head in my left hand, and crooking a finger of my right hand, I would take it out even if it meant drawing blood. Why is that? Because I have compassion for the child." 

​Buddha: "So too, prince, such speech as the Tathagata knows to be untrue, incorrect, and unbeneficial, and which is also unwelcome and disagreeable to others: such speech the Tathagata does not utter. Such speech as the Tathagata knows to be true and correct but unbeneficial, and which is also unwelcome and disagreeable to others: such speech the Tathagata does not utter. Such speech as the Tathagata knows to be true, correct, and beneficial, but which is unwelcome and disagreeable to others: the Tathagata knows the time to use such speech. Such speech as the Tathagata knows to be untrue, incorrect, and unbeneficial, but which is welcome and agreeable to others: such speech the Tathagata does not utter. Such speech as the Tathagata knows to be true and correct but unbeneficial, and which is welcome and agreeable to others: such speech the Tathagata does not utter. Such speech as the Tathagata knows to be true, correct, and beneficial, and which is welcome and agreeable to others: the Tathagata knows the time to use such speech. Why is that? Because the Tathagata has compassion for beings."

Prince: "Venerable sir, when learned nobles, learned brahmins, learned householders, and learned recluses, after formulating a question, then go to the Blessed One and pose it, has there already been in the Blessed One's mind the thought: 'If they come to me and ask me thus, I shall answer thus'? Or does that answer occur to the Tathagata on the spot?"
Buddha: "As to that, prince, I shall ask you a question in return. Answer it as you choose. What do you think, prince? Are you skilled in the parts of a chariot?"
Prince: "Yes, venerable sir, I am."
Buddha: "What do you think, prince? When people come to you and ask: 'What is the name of this part of the chariot?' has there already been in your mind the thought: 'If they come to me and ask me thus, I shall answer them thus'? Or does that answer occur to you on the spot?"
Prince: "Venerable sir, I am well known as a charioteer skilled in the parts of a chariot. All the parts of a chariot are well known to me. That answer would occur to me on the spot."
Buddha: "So too, prince, when learned nobles, learned brahmins, learned householders, and learned recluses, after formulating a question, then come to the Tathagata and pose it, the answer occurs to the Tathagata on the spot. Why is that? That element of things has been fully penetrated by the Tathagata, through the full penetration of which the answer occurs to the Tathagata on the spot."

When this was said, Prince Abhaya said: "Magnificent, venerable sir! Magnificent, venerable sir! The Blessed One has made the Dhamma clear in many ways.From today let the Blessed One remember me as a lay follower who has gone to him for refuge for life." 

 In this sutta, the Jain leader, Nigantha Nataputta, taught the Prince Abhaya a "two-horned question" to refute the Buddha's doctrine on right speech and sent the prince to challenge the Buddha. The Buddha then explained what kind of speech he would and would not say. ​The Buddha showed the factors that decide what is and is not worth saying. The main factors are three: whether or not a statement is true, whether or not it is beneficial, and whether or not it is pleasing to others. The Buddha himself would state only those things that are true and beneficial, and at the right time for when pleasing and unpleasing things should be said. Thus this discourse is not only about right speech, but also shows right speech in action.

This sutta also showed how the Buddha's response to the four categories of questions and how they should be answered. The prince asked him two questions, and in both cases he responds first with a counter-question, before going on to give an analytical answer to the first question and a categorical answer to the second. Each counter-question allowed the prince a familiar reference point for understanding the answer about to come and also to give him a chance to speak his views. 

​References: 
1. www.accesstoinsight.org
2. https://suttacentral.net/
3. The middle length discourses of the Buddha (Bhikkhu Bodhi)