Taittirīyopaniṣat 03.05

sa tapo'tapyata। sa tapastaptvā। vijñānaṃ brahmeti vyajānāt। vijñānāddhyeva khalvimāni bhūtāni jāyante। vijñānena jātāni jīvanti। vijñānaṃ prayantyabhisaṃviśantīti। tadvijñāya। punareva varuṇaṃ pitaramupasasāra। adhīhi bhagavo brahmeti। taṃ hovāca। tapasā brahma vijijñāsasva। tapo brahmeti ॥5॥

He performed austerities. After practicing austerity, he realized that "knowledge is Brahman." Indeed, it is from knowledge alone that all these beings arise; by knowledge, having been born, they live; and into knowledge they finally merge. Having understood this, he again approached his father Varuṇa and requested, "O revered one, teach me about Brahman." Varuṇa replied, "Seek to know Brahman through austerity; austerity itself is Brahman."