Brihadaranyaka Upanishad 4.3.28

4.3.28

yadvai tanna manute manvāno vai tanna manute na hi manturmaterviparilopo vidyate'vināśitvānna tu taddvitīyamasti tato'nyadvibhaktaṃ yanmanvīta

'And when (it is said that) there (in the Sushupti) he does not think, yet he is thinking, though he does not think. For thinking is inseparable from the thinker, because it cannot perish. But there is then no second, nothing else different from him that he could think.