​MN 138 Uddesavibhanga Sutta: The Exposition of a Summary 

​Thus have i heard.

On one occasion the Blessed One was living at Savatthi in Jeta's Grove, Anathapindika's Park. There the Blessed One addressed the bhikkhus thus: "Bhikkhus, I shall teach you a summary and an exposition. Listen and attend closely to what I shall say."
"Yes, venerable sir," the bhikkhus replied.
The Blessed One said this: "Bhikkhus, a bhikkhu should examine things in such a way that while he is examining them, his consciousness is not distracted and scattered externally nor stuck internally, and by not clinging he does not become agitated. If his consciousness is not distracted and scattered externally nor stuck internally, and if by not clinging he does not become agitated, then for him there is no origination of suffering - of birth, ageing, and death in the future." That is what the Blessed One said. Having said this, the Sublime One rose from his seat and went into his dwelling.

Then, soon after the Blessed One had gone, the bhikkhus considered: "Now, friends, the Blessed One has risen from his seat and gone into his dwelling after giving a summary in brief without expounding the detailed meaning. Now who will expound this in detail?" Then they considered: "The venerable Maha Kaccana is praised by the Teacher and esteemed by his wise companions in the holy life. He is capable of expounding the detailed meaning. Suppose we went to him and asked him the meaning of this." (As at Sutta 133)

Venerable Maha Kaccana: "Then listen, friends, and attend closely to what I shall say."
"Yes, friend," the bhikkhus replied. The venerable Maha Kaccana said this:  "How, friends, is consciousness called 'distracted and scattered externally'? Here, when a bhikkhu has seen a form with the eye, if his consciousness follows after the sign of form, is tied and shackled by gratification in the sign of form, is fettered by the fetter of gratification in the sign of form, then his consciousness is called 'distracted and scattered externally.' ​"

​"When he has heard a sound with the ear...smelt an odor with the nose.. .tasted a flavor with the tongue...touched a tangible with the body...cognized a mind-object with the mind, if his consciousness follows after the sign of the mind-object, is tied and shackled by gratification in the sign of the mind-object, is fettered by the fetter of gratification in the sign of the mind-object, then his consciousness is called 'distracted and scattered externally.' "

"And how, friends, is consciousness called 'not distracted and scattered externally'? Here, when a bhikkhu has seen a form with the eye, if his consciousness does not follow after the sign of form, is not tied and shackled by gratification in the sign of form, is not fettered by the fetter of gratification in the sign of form, then his consciousness is called 'not distracted and scattered externally.'  When he has heard a sound with the ear...smelt an odor with the nose...tasted a flavor with the tongue.. .touched a tangible with the body...cognized a mind-object with the mind, if his consciousness does not follow after the sign of the mind-object, is not tied and shackled by gratification in the sign of the mind-object, is not fettered by the fetter of gratification in the sign of- the mind-object, then his consciousness is called 'not distracted and scattered externally.' "

"And how, friends, is the mind called 'stuck internally'? Here, 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. If his consciousness follows after the rapture and pleasure born of seclusion, is tied and shackled by gratification in the rapture and pleasure born of seclusion, then his mind is called 'stuck internally.' Again, 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. If his consciousness follows after the rapture and pleasure born of concentration.. .then his mind is called 'stuck internally.' "

"Again, 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.' If his consciousness follows after the equanimity...then his mind is called 'stuck internally.'  Again, 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. If his consciousness follows after the neither-pain-nor-pleasure, is tied and shackled by gratification in the neither-pain-nor-pleasure, is fettered by the fetter of gratification in the neither-pain-nor-pleasure, then his mind is called 'stuck internally.' That is how the mind is called 'stuck internally.' "

"And how, friends, is the mind called 'not stuck internally'? Here, quite secluded from sensual pleasures, secluded from unwholesome states, a bhikkhu enters upon and abides in the first jhana...If his consciousness does not follow after the rapture and pleasure born of seclusion, is not tied and shackled by gratification in the rapture and pleasure born of seclusion, is not fettered by the fetter of gratification in the rapture and pleasure born of seclusion, then his mind is called 'not stuck internally.'  Again, with the stilling of applied and sustained thought, a bhikkhu enters upon and abides in the second jhana...If his consciousness does not follow after the rapture and pleasure born of concentration...then his mind is called 'not stuck internally.' "

"Again, with the fading away as well of rapture, a bhikkhu...enters upon and abides in the third jhana...If his consciousness does not follow after the equanimity...then his mind is called 'not stuck internally.' Again, with the abandoning of pleasure and pain...a bhikkhu enters upon and abides in the fourth jhana.. .If his consciousness does not follow after the neither-pain-nor-pleasure, is not tied and shackled by gratification in the neither-pain-nor-pleasure, is not fettered by the fetter of gratification in the neither-pain-nor-pleasure, then his mind is called 'not stuck internally.' That is how the mind is called 'not stuck internally.' "

"How, friends, is there agitation due to clinging? Here an untaught ordinary person who has no regard for noble ones and is unskilled and undisciplined in their Dhamma, who has no regard for true men and is unskilled and undisciplined in their Dhamma, regards material form as self, or self as possessed of material form, or material form as in self, or self as in material form. That material form of his changes and becomes otherwise. With the change and becoming otherwise of that material form, his consciousness is preoccupied with the change of material form. Agitated mental states born of preoccupation with the change of material form arise together and remain obsessing his mind. Because his mind is obsessed, he is anxious, distressed, and concerned, and due to clinging he becomes agitated."

"He regards feeling as self...He regards perception as self...He regards formations as self...He regards consciousness as self, or self as possessed of consciousness, or consciousness as in self, or self as in consciousness. That consciousness of his changes and becomes otherwise. With the change and becoming otherwise of that consciousness, his consciousness is preoccupied with the change of consciousness. Agitated states of mind born of preoccupation with the change of consciousness arise together and remain obsessing his mind. Because his mind is obsessed, he is anxious, distressed, and concerned, and due to clinging he becomes agitated. That is how there is agitation due to clinging."

"And how, friends, is there non-agitation due to non-clinging? Here a well-taught noble disciple who has regard for noble ones and is skilled and disciplined in their Dhamma, who has regard for true men and is skilled and disciplined in their Dhamma, does not regard material form as self, or self as possessed of material form, or material form as in self, or self as in material form. That material form of his changes and becomes otherwise. With the change and becoming otherwise of that material form, his consciousness is not preoccupied with the change of material form. Agitated mental states born of preoccupation with the change of material form do not arise together ​and remain obsessing his mind. Because his mind is not obsessed, he is not anxious, distressed, and concerned, and due to non-clinging he does not become agitated. "

​"He does not regard feeling as self...He does not regard perception as self.. .He does not regard formations as self.. .He does not regard consciousness as self, or self as possessed of consciousness, or consciousness as in self, or self as in consciousness. That consciousness of his changes and becomes otherwise. With the change and becoming otherwise of that consciousness, his consciousness is not preoccupied with the change of consciousness. Agitated mental states born of preoccupation with the change of consciousness do not arise together and remain obsessing his mind. Because his mind is not obsessed, he is not anxious, distressed, and concerned, and due to non-clinging he does not become agitated. That is how there is non-agitation due to non-clinging. "

"Friends, when the Blessed One rose from his seat and went into his dwelling after giving a summary in brief without expounding the detailed meaning, that is: 'Bhikkhus, a bhikkhu should examine things in such a way that while he is examining them, his consciousness is not distracted and scattered externally nor stuck internally, and by not clinging he does not become agitated. If his consciousness is not distracted and scattered externally nor stuck internally, and if by not clinging he does not become agitated, then for him there is no origination of suffering - of birth, ageing, and death in the future,' I understand the detailed meaning of this summary to be thus. Now, friends, if you wish, go to the Blessed One and ask him about the meaning of this. As the Blessed One explains it to you, so you should remember it."

Then the bhikkhus, having delighted and rejoiced in the venerable Maha Kaccana's words, rose from their seats and went to the Blessed One. After paying homage to him, they sat down at one side and told the Blessed One all that had taken place after he had left, adding: "Then, venerable sir, we went to the venerable Maha Kaccana and asked him about the meaning. The venerable Maha Kaccana expounded the meaning to us with these terms, statements, and phrases."

Buddha: "Maha Kaccana is wise, bhikkhus, Maha Kaccana has great wisdom. If you had asked me the meaning of this, I would have explained it to you in the same way that Maha Kaccana has explained it. Such is its meaning, and so you should remember it." 

That is what the Blessed One said. The bhikkhus were satisfied and delighted in the Blessed One's words. 

In summary, the Buddha advises the bhikkhus that when investigating things one's consciousness should not be allowed to wander in such a way as to allow thoughts supporting further existence whether that be of externals such as sense experience or of internals such as the factors of the jhanas. He said to the monks :" Bhikkhus, a bhikkhu should examine things in such a way that while he is examining them, his consciousness is not distracted and scattered externally nor stuck internally, and by not clinging he does not become agitated. If his consciousness is not distracted and scattered externally nor stuck internally, and if by not clinging he does not become agitated, then for him there is no origination of suffering - of birth, ageing, and death in the future."

The monks then approach Venerable Maha Kaccana to elaborate upon a brief saying of the Buddha on the training of consciousness and the overcoming of agitation. ​

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