Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 4.3.26

4.3.26

yadvai tanna vadati vadanvai tanna vadati na hi vakturvaktervipavilopo vidyate'vināśitvānna tu taddvitīyamasti tato'nyadvibhaktaṃ yadvadet

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