BṚHADĀRAṆYAKA UPANISHAD · 4.3.28

यद् वै तन् न मनुते मन्वानो वै तन् न मनुते ।
न हि मन्तुर् मतेर् विपरिलोपो विद्यते ऽविनाशित्वात् ।
न तु तद् द्वितीयम् अस्ति ततो ऽन्यद् विभक्तं यन् मन्वीत ॥

yad vai tan na manute manvāno vai tan na manute ।
na hi mantur mater viparilopo vidyate ’vināśitvāt ।
na tu tad dvitīyam asti tato ’nyad vibhaktaṃ yan manvīta ॥

'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.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)