Muṇḍaka Upaniṣad 1.1.3

1.1.3

śaunako ha vai mahāśālo 'ṅgirasaṃ vidhivad upasannaḥ papraccha.
kasmin nu bhagavo vijñāte sarvam idaṃ vijñātaṃ bhavatīti.

3. Śaunaka, verily, indeed, a great householder, approached Aṅgiras according to rule, and asked: ‘Through understanding of what, pray, does all this world become understood, Sir?’