At Sāvatthī.
Seated to one side, Venerable Rādha said to the Buddha:
“Sir, they speak of this thing called a ‘sentient being’. How is a sentient being defined?”
“Rādha, when you cling, strongly cling, to desire, greed, relishing, and craving for form, then a being is spoken of. When you cling, strongly cling, to desire, greed, relishing, and craving for feeling … perception … choices … consciousness, then a being is spoken of.
Suppose some boys or girls were playing with sandcastles. As long as they’re not rid of greed, desire, fondness, thirst, passion, and craving for those sandcastles, they cherish them, fancy them, treasure them, and treat them as their own. But when they are rid of greed, desire, fondness, thirst, passion, and craving for those sandcastles, they scatter, destroy, and demolish them with their hands and feet, making them unplayable.
In the same way, you should scatter, destroy, and demolish form, making it unplayable. And you should practice for the ending of craving. You should scatter, destroy, and demolish feeling … perception … choices … consciousness, making it unplayable. And you should practice for the ending of craving. For the ending of craving is extinguishment.”

SN 23.2

On arrival, they saw the Blessed One sitting in the grove — his legs folded crosswise, his body set straight, mindfulness established to the fore. On seeing him, they went to the brahman of the Bharadvaja clan and, on arrival, said to him, “Sir, you should know that Gotama the contemplative is in that grove over there, sitting with his legs folded crosswise, his body set straight, mindfulness established to the fore.”
So the brahman of the Bharadvaja clan, together with the youths, went to the forest grove. On arrival, he saw the Blessed One sitting in the grove — his legs folded crosswise, his body set straight, mindfulness established to the fore. On seeing him, he went to the Blessed One and, on arrival, addressed him in verse:

In the deep-looking forest,
teeming with terrors,
having plunged into the wilderness
— desolate, empty —
unflinchingly, steadfastly, compellingly,
you practice jhana, monk:
How very lovely you look!

SN 7.18

Then Ven. Maha Kotthita went to Ven. Sariputta and, on arrival, exchanged courteous greetings with him. After an exchange of friendly greetings & courtesies, he sat to one side. As he was sitting there, he said to Ven. Sariputta, “With the remainderless stopping & fading of the six contact-media [vision, hearing, smell, taste, touch, & intellection] is it the case that there is anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “With the remainderless stopping & fading of the six contact-media, is it the case that there is not anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “…is it the case that there both is & is not anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “…is it the case that there neither is nor is not anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “Being asked if, with the remainderless stopping & fading of the six contact-media, there is anything else, you say, ‘Don’t say that, my friend.’ Being asked if … there is not anything else … there both is & is not anything else … there neither is nor is not anything else, you say, ‘Don’t say that, my friend.’ Now, how is the meaning of your words to be understood?”
[Sariputta:] “The statement, ‘With the remainderless stopping & fading of the six contact-media [vision, hearing, smell, taste, touch, & intellection] is it the case that there is anything else?’ objectifies non-objectification. The statement, ‘… is it the case that there is not anything else … is it the case that there both is & is not anything else … is it the case that there neither is nor is not anything else?’ objectifies non-objectification. However far the six contact-media go, that is how far objectification goes. However far objectification goes, that is how far the six contact media go. With the remainderless fading & stopping of the six contact-media, there comes to be the stopping, the allaying of objectification.
“And suppose someone were to ask you, ‘This fire that has gone out in front of you, in which direction from here has it gone? East? West? North? Or south?’ Thus asked, how would you reply?”
“That doesn’t apply, Master Gotama. Any fire burning dependent on a sustenance of grass and timber, being unnourished — from having consumed that sustenance and not being offered any other — is classified simply as ‘out’ (unbound).”
“Even so, Vaccha, any physical form by which one describing the Tathagata would describe him: That the Tathagata has abandoned, its root destroyed, made like a palmyra stump, deprived of the conditions of development, not destined for future arising. Freed from the classification of form, Vaccha, the Tathagata is deep, boundless, hard to fathom, like the sea. ‘Reappears’ doesn’t apply. ‘Does not reappear’ doesn’t apply. ‘Both does & does not reappear’ doesn’t apply. ‘Neither reappears nor does not reappear’ doesn’t apply.

When we feel the urge to reflect on qualities, motives of the Enlightened Being, when we try to envisage what his thought and emotions look like, we always should keep in mind that such a being is transcendent, indescribable and cannot be grasped by us.

At one time Venerable Sabhiya Kaccāna was staying at Nādika in the brick house. Then the wanderer Vacchagotta went up to him, and exchanged greetings with him. When the greetings and polite conversation were over, he sat down to one side, and said to Sabhiya Kaccāna:
“Master Kaccāna, does a Realized One exist after death?”
“Vaccha, this has not been declared by the Buddha.”
“Well then, does a Realized One not exist after death?”
“This too has not been declared by the Buddha.”
“Well then, does a Realized One both exist and not exist after death?”
“This has not been declared by the Buddha.”
“Well then, does a Realized One neither exist nor not exist after death?”
“This too has not been declared by the Buddha.”
“Master Kaccāna, when asked these questions, you say that this has not been declared by the Buddha. What’s the cause, what’s the reason why this has not been declared by the Buddha?”
“In order to describe him as ‘possessing form’ or ‘formless’ or ‘percipient’ or ‘non-percipient’ or ‘neither percipient nor non-percipient’, there must be some cause or reason for doing so. But if that cause and reason were to totally and utterly cease without anything left over, how could you describe him in any such terms?”
“Master Kaccāna, how long has it been since you went forth?”
“Not long, reverend: three years.”
“Well, you’ve learned a lot already, let alone what lies ahead!”

SN 44.11

There is, monks, an unborn — unbecome — unmade — unfabricated. If there were not that unborn — unbecome — unmade — unfabricated, there would not be the case that emancipation from the born — become — made — fabricated would be discerned. But precisely because there is an unborn — unbecome — unmade — unfabricated, emancipation from the born — become — made — fabricated is discerned.

“Mālunkyāputta, if there is the view ‘the world is eternal,’ the holy life cannot be lived; and if there is the view ‘the world is not eternal,’ the holy life cannot be lived. Whether there is the view ‘the world is eternal’ or the view ‘the world is not eternal,’ there is birth, there is ageing, there is death, there are sorrow, lamentation, pain, grief, and despair, the destruction of which I prescribe here and now.

“If there is the view ‘the world is finite,’…‘the world is infinite, ’…‘the soul is the same as the body,’…‘the soul is one thing and the body another,’…‘after death a Tathāgata exists,’…‘after death a Tathāgata does not exist,’ the holy life cannot be lived… If there is the view ‘after death a Tathāgata both exists and does not exist,’ the holy life cannot be lived; and if there is the view ‘after death a Tathāgata neither exists nor does not exist,’ the holy life cannot be lived. Whether there is the view ‘after death a Tathāgata both exists and does not exist’ or the view ‘after death a Tathāgata neither exists nor does not exist,’ there is birth, there is ageing, there is death, there are sorrow, lamentation, pain, grief, and despair, the destruction of which I prescribe here and now.

“Therefore, Mālunkyāputta, remember what I have left undeclared as undeclared, and remember what I have declared as declared. And what have I left undeclared? ‘The world is eternal’—I have left undeclared. ‘The world is not eternal’—I have left undeclared. ‘The world is finite’—I have left undeclared. ‘The world is infinite’—I have left undeclared. ‘The soul is the same as the body’—I have left undeclared. ‘The soul is one thing and the body another’—I have left undeclared. ‘After death a Tathāgata exists’—I have left undeclared. ‘After death a Tathāgata does not exist’—I have left undeclared. ‘After death a Tathāgata both exists and does not exist’—I have left undeclared. ‘After death a Tathāgata neither exists nor does not exist’—I have left undeclared.

“Why have I left that undeclared? Because it is unbeneficial, it does not belong to the fundamentals of the holy life, it does not lead to disenchantment, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna. That is why I have left it undeclared.

MN 63