​MN 62 Maharahulovada Sutta: The Greater Discourse of Advice to Rahula 

Thus have i heard.

​On one occasion the Blessed One was living at Savatthi in Jeta's Grove, Anathapindika's Park. In the morning, the Blessed One dressed, and taking his bowl and outer robe, went into Savatthi for alms. The venerable Rahula also got dressed, and taking his bowl and outer robe, followed close behind the Blessed One.

Then the Blessed One looked back and addressed the venerable Rahula thus: "Rahula, any kind of material form whatever, whether past, future, or present, internal or external, gross or subtle, inferior or superior, far or near, all material form should be seen as it actually is with proper wisdom thus: 'This is not mine, this I am not, this is not my self.'"
Rahula: "Only material form, Blessed One? Only material form, Sublime One?"
Buddha: "Material form, Rahula, and feeling, perception, formations, and consciousness."

Then the venerable Rahula considered thus: "Who would go into the town for alms today when personally admonished by the Blessed One?" Thus he turned back and sat down at the root of a tree, folding his legs crosswise, setting his body erect, and establishing mindfulness in front of him. Venerable Sariputta saw him sitting there and addressed him thus: "Rahula, develop mindfulness of breathing. When mindfulness of breathing is developed and cultivated, it is of great fruit and great benefit."

Then, when it was evening, the venerable Rahula rose from meditation and went to the Blessed One. After paying homage to him, he sat down at one side and asked the Blessed One: "Venerable sir, how is mindfulness of breathing developed and cultivated, so that it is of great fruit and great benefit?" 

THE FOUR GREAT ELEMENTS

"Rahula, whatever internally, belonging to oneself which is solid, solidified, and clung-to, that is, head-hairs, body-hairs, nails, teeth, skin, flesh,bones, bone-marrow, organs, intestines, contents of the stomach, feces, or whatever else internally, belonging to oneself: this is called the internal earth element. Now both the internal earth element and the external earth element are simply earth element. And that should be seen as it actually is with proper wisdom thus: 'This is not mine, this I am not, this is not my self.'  When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the earth element and makes the mind dispassionate towards the earth element. "

"What, Rahula, is the water element? The water element may be either internal or external. Whatever internally, belonging to oneself, is water, watery, and clung-to, that is, bile, phlegm, pus, blood, sweat, fat, tears, grease, snot, cartilage, urine, or whatever else internally, belonging to oneself: this is called the internal water element. Now both the internal water element and the external water element are simply water element. And that should be seen as it actually is with proper wisdom thus: 'This is not mine, this I am not, this is not my self.' When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the water element and makes the mind dispassionate towards the water element. "

"What, Rahula, is the fire element? The fire element may be either internal or external. What is the internal fire element? Whatever internally, belonging to oneself, is fire, fiery, and clung-to, that is, that by which one is warmed, ages, and is consumed, and that by which what is eaten, drunk, consumed, and tasted gets completely digested, or whatever else internally, belonging to oneself: this is called the internal fire element. Now both the internal fire element and the external fire element are simply fire element. And that should be seen as it actually is with proper wisdom thus: 'This is not mine, this I am not, this is not my self.' When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the fire element and makes the mind dispassionate towards the fire element. ​"

 "What, Rahula, is the air element? The air element may be either internal or external. What is the internal air element? Whatever internally, belonging to oneself, is air, airy, and clung-to, that is, up-going winds, down-going winds, winds in the belly, winds in the bowels, winds that course through the limbs, in-breath and out-breath, or whatever else internally, belonging to oneself: this is called the internal air element. Now both the internal air element and the external air element are simply air element. And that should be seen as it actually is with proper wisdom thus: 'This is not mine, this I am not, this is not my self.' When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the air element and makes the mind dispassionate towards the air element." 

"What, Rahula, is the space element? The space element may be either internal or external. What is the internal space element? Whatever internally, belonging to oneself, is space, spatial, and clung-to, that is, space such as openings of the ears, the nose, the mouth, the space in which is swallowed the food we have eaten, the space where the food is held, and the space it passes through to be excreted through the lower part of the body, or whatever else internally: this is called the internal space element. Now both the internal space element and the external space element are simply space element. And that should be seen as it actually is with proper wisdom thus: 'This is not mine, this I am not, this is not my self.' When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the space element and makes the mind dispassionate towards the space element. "

"Rahula, develop meditation that is like the earth; for when you develop meditation that is like the earth, arisen pleasant and unpleasant sensations will not invade your mind and remain. Just as people throw clean things and dirty things like excrement, urine, pus, and blood on the earth, and the earth is not horrified, humiliated, and disgusted because of that, so too, Rahula, develop meditation that is like the earth; for when you develop meditation that is like the earth, arisen pleasant and unpleasant sensations will not invade your mind and remain."

​"Rahula, develop meditation that is like water; for when you develop meditation that is like water, arisen pleasant and unpleasant sensations will not invade your mind and remain. Just as people wash clean things and dirty things, excrement, urine, pus, and blood in water, and the water is not horrified, humiliated, and disgusted because of that, so too Rahula, develop meditation that is like water; for when you develop meditation that is like water, arisen agreeable and disagreeable contacts will not invade your mind and remain."

"Rahula, develop meditation that is like fire/air/space; for when you develop meditation that is like fire/air/space, arisen pleasant and unpleasant sensations will not invade your mind and remain. Just as people bum clean things and dirty things in fire, and the fire/air/space is not horrified, humiliated, and disgusted because of that, so too, Rahula, develop meditation that is like fire/air/space; for when you develop meditation that is like fire/air/space, arisen agreeable and disagreeable contacts will not invade your mind and remain."

"Rahula, develop meditation on loving-kindness; for when you develop meditation on loving-kindness, any ill will will be abandoned. Develop meditation on compassion; for when you develop meditation on compassion, any cruelty will be abandoned. Develop meditation on appreciative joy; for when you develop meditation on appreciative joy, any discontent will be abandoned. Develop meditation on equanimity; for when you develop meditation on equanimity, any aversion will be abandoned. Develop meditation on foulness; for when you develop meditation on foulness, any lust will be abandoned. Develop meditation on the perception of impermanence; for when you develop meditation on the perception of impermanence, the conceit 'I am' will be abandoned. "

"Rahula, develop meditation on mindfulness of breathing. When mindfulness of breathing is developed and cultivated, it is of great fruit and great benefit. And how is mindfulness of breathing developed and cultivated, so that it is of great fruit and great benefit? Here, Rahula, a bhikkhu, gone to the forest or to the root of a tree or to an empty hut, sits down; having folded his legs crosswise, set his body erect, and established mindfulness in front of him, ever mindful he breathes in, mindful he breathes out. Breathing in long, he understands: 'I breathe in long'; or breathing out long, he understands: 'I breathe out long.' Breathing in short, he understands: 'I breathe in short'; or breathing out short, he understands: 'I breathe out short.' He trains thus: 'I shall breathe in experiencing the whole body of breath'; he trains thus: 'I shall breathe out experiencing the whole body of breath.' He trains thus: 'I shall breathe in tranquilizing the bodily formation'; he trains thus: 'I shall breathe out tranquilizing the bodily formation.' "

"He trains thus: 'I shall breathe in experiencing rapture'; he trains thus: 'I shall breathe out experiencing rapture.' He trains thus: 'I shall breathe in experiencing pleasure'; he trains thus: 'I shall breathe out experiencing pleasure.' He trams thus: 'I shall breathe in experiencing the mental formation'; he trains thus: 'I shall breathe out experiencing the mental formation.' He trams thus: 'I shall breathe in tranquilizing the mental formation'; he trams thus: 'I shall breathe out tranquilizing the mental formation.' "

 "He trains thus: T shall breathe in experiencing the mind'; he trains thus: 'I shall breathe out experiencing the mind-' He trains thus: 'I shall breathe in gladdening the mind'; life trains thus: 'I shall breathe out gladdening the mind.' He trains thus: 'I shall breathe in concentrating the mind'; he trains thus: 'I shall breathe out concentrating the mind.' He trains thus: 'I shall breathe in liberating the mind'; he trains thus: 'I shall breathe out liberating the mind.' "

"He trains thus: 'I shall breathe in contemplating impermanence'; he trains thus: 'I shall breathe out contemplating impermanence.' He trains thus: 'I shall breathe in contemplating fading away'; he trains thus: 'I shall breathe out contemplating fading away.' He trains thus; 'I shall breathe in contemplating cessation'; he trains thus: I shall breathe out contemplating cessation.' He trains thus: 'I shall breathe in contemplating relinquishment'; he trains thus: 'I shall breathe out contemplating relinquishment.' "

"Rahula, that is how mindfulness of breathing is developed and cultivated, so that it is of great fruit and great benefit. When mindfulness of breathing is developed and cultivated in this way, even the final in-breaths and out-breaths are known as they cease, not unknown."

That is what the Blessed One said. The venerable Rahula was satisfied and delighted in the Blessed One's words. 

The Buddha taught Rāhula (his son who was a novice monk) that all five aggregates (khandas) should be regarded as non-self (anattā) and not only (form) rūpa. When Rāhula sat under a tree meditating, Venerable Sāriputta approached him and suggested that he should develop meditation on contemplation of breathing (ānāpānasati). Later in the evening Rāhula asked the Buddha how he can do this. The Buddha then described how it is done by regarding all the five elements   earth, water, fire, air and space. One should not allow sensory pleasant and unpleasant contacts to lay hold of one's mind just as how the elements remains impassive whatever may be thrown upon it. One should grow in loving-kindness, compassion, appreciative joy, in equanimity, contemplation of the body's corruption, perception of the impermanence nature of things, and in the mindfulness which comes from ordered breathing. That's how the Buddha taught Rahula the meditation on the elements, on mindfulness of breathing and other topics. 

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