​MN 119 Kayagatasati Sutta: Mindfulness of the Body 

Thus have i heard.

On one occasion the Blessed One was living at Savatthi in Jeta's Grove, Anathapindika's Park. Now a number of bhikkhus were sitting in the assembly hall, where they had met together upon returning from their alms round, after their meal, when this discussion arose among them: "It is wonderful, friends, it is marvellous, how it has been said by the Blessed One who knows and sees, accomplished and fully enlightened, that mindfulness of the body, when developed and cultivated, is of great fruit and great benefit."

However, their discussion was interrupted; for the Blessed One rose from meditation when it was evening, went to the assembly hall, and sat down on a seat made ready. Then he addressed the bhikkhus thus: "Bhikkhus, what discussion are you engaged in?"
They replied: "This discussion arose among us: 'It is wonderful, friends, it is marvellous, how it has been said by the Blessed One who knows and sees, accomplished and fully enlightened, that mindfulness of the body, when developed and cultivated, is of great fruit and great benefit.' This was our discussion, venerable sir, that was interrupted when the Blessed One arrived."

Buddha: "And how, bhikkhus, is mindfulness of the body developed and cultivated so that it is of great fruit and great benefit? "

MINDFULNESS OF BREATHING

"Here 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.' As he abides thus diligent, ardent, and resolute, his memories and intentions based on the household life are abandoned; with their abandoning his mind becomes steadied internally, quieted, brought to singleness, and concentrated. That is how a bhikkhu develops mindfulness of the body."
THE FOUR POSTURES

​"Again, bhikkhus, when walking, a bhikkhu understands: 'I am walking'; when standing, he understands: 'I am standing'; when sitting, he understands: 'I am sitting'; when lying down, he understands: 'I am lying down'; or he understands accordingly however his body is disposed. As he abides thus diligent, ardent, and resolute, his memories and intentions based on the household life are abandoned...That too is how a bhikkhu develops mindfulness of the body.

FULL AWARENESS

"Again, bhikkhus, a bhikkhu is one who acts in full awareness when going forward and returning; who acts in full awareness when looking ahead and looking away; who acts in full awareness when flexing and extending his limbs; who acts in full awareness when wearing his robes and carrying his outer robe and bowl; who acts in full awareness when eating, drinking, consuming food, and tasting; who acts in full awareness when defecating or urinating; who acts in full awareness when walking, standing, sitting, falling asleep, waking up, talking, and keeping silent. As he abides thus diligent, ardent, and resolute, his memories and intentions based on the household life are abandoned...That too is how a bhikkhu develops mindfulness of the body. "
FOULNESS - THE BODILY PARTS

"Again, bhikkhus, a bhikkhu reviews this same body up from the soles of the feet and down from the top of the hair, bounded by skin, as full of many kinds of impurity. Just as though there were a bag with an opening at both ends full of many sorts of grain and a man with good eyes were to open it and review it thus: 'This is hill rice, this is red rice, these are ...'; so too, a bhikkhu reviews this same body as full of many kinds of impurity thus: 'In this body there are head-hairs...and urine.' As he abides thus diligent, ardent, and resolute, his memories and intentions based on the household life are abandoned...That too is how a bhikkhu develops mindfulness of the body.

ELEMENTS

"Again, bhikkhus, a bhikkhu reviews this same body, however it is placed, however disposed, as consisting of elements thus: 'In this body there are the earth element, the water element, the fire element, and the air element.' Just as though a skilled butcher had dissected a cow into pieces; so too, a bhikkhu reviews this same body however it is placed, however disposed, as consisting of elements thus: In this body there are the earth element, the water element, the fire element, and the air element.' As he abides thus diligent, ardent, and resolute, his memories and intentions connected with the household life are abandoned... That too is how a bhikkhu develops mindfulness of the body." 
THE NINE CHARNEL GROUND CONTEMPLATION

"Again, bhikkhus, as though he were to see a corpse thrown aside in a charnel ground, one, two, or three days dead, bloated, livid, and oozing matter, a bhikkhu compares this same body with it thus: 'This body too is of the same nature, it will be like that, it is not exempt from that fate.' As he abides thus diligent... That too is how a bhikkhu develops mindfulness of the body. Again, as though he were to see a corpse thrown aside in a charnel ground, being devoured by crows, hawks, vultures, dogs, jackals, or various kinds of worms, a bhikkhu compares this same body with it thus: 'This body too is of the same nature, it will be like that, it is not exempt from that fate.' As he abides thus diligent...That too is how a bhikkhu develops mindfulness of the body."

"Again, as though he were to see a corpse thrown aside in a charnel ground, a skeleton with flesh and blood, held together with sinews...a flesh-less skeleton smeared with blood, held together with sinews...a skeleton without flesh and blood, held together with sinews...disconnected bones scattered in all directions- a bhikkhu compares this same body with it thus: 'This body too is of the same nature, it will be like that, it is not exempt from that fate.' As he abides thus diligent...That too is how a bhikkhu develops mindfulness of the body. Again, as though he were to see a corpse thrown aside in a charnel ground, bones bleached white, ...bones heaped up, more than a year old...bones rotted and crumbled to dust, a bhikkhu compares this same body with it thus: 'This body too is of the same nature, it will be like that, it is not exempt from that fate.' As he abides thus diligent.. .That too is how a bhikkhu develops mindfulness of the body. 

THE JHANAS

"Again, bhikkhus, quite secluded from sensual pleasures, secluded from unwholesome states, a bhikkhu enters upon and abides in the first jhana, which is accompanied by applied and sustained thought, with rapture and pleasure born of seclusion. He makes the rapture and pleasure born of seclusion pervade this body, so that there is no part of his whole body un-pervaded by the rapture and pleasure born of seclusion. As he abides thus diligent...That too is how a bhikkhu develops mindfulness of the body."

"Again, bhikkhus, with the stilling of applied and sustained thought, a bhikkhu enters upon and abides in the second jhana, which has self-confidence and singleness of mind without applied and sustained thought, with rapture and pleasure born of concentration. He makes the rapture and pleasure born of concentration pervade this body, so that there is no part of his whole body un-pervaded by the rapture and pleasure born of concentration. As he abides thus diligent...That too is how a bhikkhu develops mindfulness of the body."

"Again, bhikkhus, with the fading away as well of rapture, a bhikkhu abides in equanimity, and mindful and fully aware, still feeling pleasure with the body, he enters upon and abides in the third jhana, on account of which noble ones announce: 'He has a pleasant abiding who has equanimity and is mindful.' He makes the pleasure divested of rapture  pervade this body, so that there is no part of his whole body un-pervaded by the pleasure divested of rapture. As he abides thus diligent...That too is how a bhikkhu develops mindfulness of the body."
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"Again, bhikkhus, with the abandoning of pleasure and pain, and with the previous disappearance of joy and grief, a bhikkhu enters upon and abides in the fourth jhana, which has neither-pain-nor-pleasure and purity of mindfulness due to equanimity. He sits pervading this body with a pure bright mind, so that there is no part of his whole body un-pervaded by the pure bright mind. As he abides thus diligent, ardent, and resolute, his memories and intentions based on the household life are abandoned; with their abandoning his mind becomes steadied internally, quieted, brought to singleness, and concentrated. That too is how a bhikkhu develops mindfulness of the body. "
PROGRESS THROUGH MINDFULNESS OF THE BODY

"Bhikkhus, anyone who has developed and cultivated mindfulness of the body has included within himself whatever wholesome states there are that partake of true knowledge.Just as anyone who has extended his mind over the great ocean has included within it whatever streams there are that flow into the ocean; so too, anyone who has developed and cultivated mindfulness of the body has included within himself whatever wholesome states there are that partake of true knowledge. "

"Bhikkhus, when anyone has not developed and cultivated mindfulness of the body, Mara finds an opportunity and a support in him. Suppose a man were to throw a heavy stone ball upon a mound of wet clay. What do you think, bhikkhus? Would that heavy ball find entry into that mound of wet clay?"
"Yes, venerable sir."
​"So too, bhikkhus, when anyone has not developed and cultivated mindfulness of the body, Mara finds an opportunity and a support in him."

"Suppose there were a dry sapless piece of wood, and a man came with an upper fire-stick, thinking: 'I shall light a fire, I shall produce heat.' What do you think, bhikkhus? Could the man light a fire and produce heat by rubbing the dry sapless piece of wood with an upper fire-stick?"
"Yes, venerable sir."
"So too, bhikkhus, when anyone has not developed and cultivated mindfulness of the body, Mara finds an opportunity and a support in him."

"Suppose there were a hollow empty water jug set out on a stand, and a man came with a supply of water. What do you think, bhikkhus? Could the man pour the water into the jug?"
"Yes, venerable sir."
"So too, bhikkhus, when anyone has not developed and cultivated mindfulness of the body, Mara finds an opportunity and a support in him."
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"Bhikkhus, when anyone has developed and cultivated mindfulness of the body, Mara cannot find an opportunity or a support in him. Suppose a man were to throw a light ball of string at a door-panel made entirely of heartwood. What do you think, bhikkhus? Would that light ball of string find entry through that door-panel made entirely of heartwood?"
"No, venerable sir."
"So too, bhikkhus, when anyone has developed and cultivated mindfulness of the body, Mara cannot find an opportunity or a support in him."

"Suppose there were a wet sappy piece of wood, and a man came with an upper fire-stick, thinking: 'I shall light a fire, I shall produce heat.' What do you think, bhikkhus? Could the man light a fire and produce heat by taking the upper fire stick and rubbing it against the wet sappy piece of wood?"
"No, venerable sir."
​"So too, bhikkhus, when anyone has developed and cultivated mindfulness of the body, Mara cannot find an opportunity or a support in him. "
​"Suppose, set out on a stand, there were a water jug full of water right up to the brim so that crows could drink from it, and a man came with a supply of water. What do you think, bhikkhus? Could the man pour the water into the jug?"
"No, venerable sir."
"So too, bhikkhus, when anyone has developed and cultivated mindfulness of the body, Mara cannot find an opportunity or a support in him."

"Bhikkhus, when anyone has developed and cultivated mindfulness of the body, then when he inclines his mind towards realizing any state that may be realized by direct knowledge, he attains the ability to witness any aspect therein, there being a suitable basis. Suppose, set out on a stand, there were a water jug full of water right up to the brim so that crows could drink from it. Whenever a strong man tips it, would water come out?"
"Yes, venerable sir."
"So too, bhikkhus, when anyone has developed and cultivated mindfulness of the body, then when he inclines his mind towards realizing any state that may be realized by direct knowledge, he attains the ability to witness any aspect therein, there being a suitable basis."

"Suppose there were a square pond on level ground, surrounded by an embankment, full of water right up to the brim so that crows could drink from it. Whenever a strong man loosens the embankment, would water come out?" - "Yes, venerable sir."
"So too, bhikkhus, when anyone has developed and cultivated mindfulness of the body.. .he attains the ability to witness any aspect therein, there being a suitable basis."

"Suppose there were a chariot on even ground at the crossroads, harnessed to thoroughbreds, waiting with goad lying ready, so that a skilled trainer, a charioteer of horses to be tamed, might mount it, and taking the reins in his left hand and the goad in his right hand, might drive out and back by any road whenever he likes. So too, bhikkhus, when anyone has developed and cultivated mindfulness of the body.. .he attains the ability to witness any aspect therein, there being a suitable basis. "

BENEFITS OF MINDFULNESS OF THE BODY

"Bhikkhus, when mindfulness of the body has been repeatedly practiced, developed, cultivated, used as a vehicle, used as a basis, established, consolidated, and well undertaken, these ten benefits may be expected. What ten? 
(1) One becomes a conqueror of discontent and delight, and discontent does not conquer oneself; one abides overcoming discontent whenever it arises.
(2) One becomes a conqueror of fear and dread, and fear and dread do not conquer oneself; one abides overcoming fear and dread whenever they arise. 
(3) One bears cold and heat, hunger and thirst, and contact with gadflies, mosquitoes, wind, the sun, and creeping things; one endures ill-spoken, unwelcome words and arisen bodily feelings that are painful, racking, sharp, piercing, disagreeable, distressing, and menacing to life.
(4) One obtains at will, without trouble or difficulty, the four jhanas that constitute the higher mind and provide a pleasant abiding here and now.

(5) One wields the various kinds of super-normal power... (as Sutta 108)...one wields bodily mastery even as far as the Brahma-world. 
(6) With the divine ear element, which is purified and surpasses the human, one hears both kinds of sounds, the divine and the human, those that are far as well as near.
(7) One understands the minds of other beings, of other persons, having encompassed them with one's own mind. One understands a mind affected by lust as affected by lust...(as Sutta 108)...an un-liberated mind as un-liberated.
(8) One recollects one's manifold past lives, that is...(as Sutta 51)...Thus with their aspects and particulars one recollects one's manifold past lives.

(9) With the divine eye, which is purified and surpasses the human, one sees beings passing away and reappearing, inferior and superior, fair and ugly, fortunate and unfortunate, and one understands how beings pass on according to their actions
(10) By realizing for oneself with direct knowledge, one here and now enters upon and abides in the deliverance of mind and deliverance by wisdom that are taint-less with the destruction of the taints.

"Bhikkhus, when mindfulness of the body has been repeatedly practiced, developed, cultivated, used as a vehicle, used a basis, established, consolidated, and well undertaken, these ten benefits may be expected." 

That is what the Blessed One said. The bhikkhus were satisfied and delighted in the Blessed One's words. 
In summary, the Buddha explains how mindfulness of the body should be developed and cultivated and the ten benefits to which it leads. Buddha first taught the monks mindfulness of the breathing followed by constant awareness of the body's posture and positions. Then he taught them to reflect on the repulsiveness of the body, contemplation on the impermanence of the body and death by contemplating the human corpses in different states of decomposition. Then he explained the four states of Jhanas attainments that can be reached through meditation. Then Buddha concluded the discourse by mentioning the ten benefits from practicing mindfulness of the body.  
References: 
1. www.accesstoinsight.org
2. https://suttacentral.net/
3. The middle length discourses of the Buddha (Bhikkhu Bodhi)