Brihadaranyaka Upanishad 3.4.2.p1

3.4.2.p1

iti tṛtīyādhyāye caturthaṃ brāhmaṇam

Ushasta, the son of Chakra, said, ‘You have indicated it as one may say that a cow is such and such, or a horse is such and such. Explain to me the Brahman that is immediate and direct—the self that is within all.’ ‘This is your self that is within all.’ ‘Which is within all, Yâjnavalkya?’ ‘You cannot see that which is the witness of vision; you cannot hear that which is the hearer of hearing; you cannot think that which is the thinker of thought; you cannot know that which is the knower of knowledge. This is your self that is within all; everything else but this is perishable.’ Thereupon Ushasta, the son of Chakra, kept silent.