Taittirīyopaniṣat 03.03

sa tapo'tapyata। sa tapastaptvā। prāṇo brahmeti vyajānāt। prāṇāddhyeva khalvimāni bhūtāni jāyante। prāṇena jātāni jīvanti। prāṇaṃ prayantyabhisaṃviśantīti। tadvijñāya। punareva varuṇaṃ pitaramupasasāra। adhīhi bhagavo brahmeti। taṃ hovāca। tapasā brahma vijijñāsasva। tapo brahmeti ॥3॥

He practiced austerity. Having performed austerity, he realized that "prāṇa is Brahman." Indeed, from prāṇa alone all these beings are born; by prāṇa, the born live; into prāṇa they depart and merge. Having understood this, he again approached his father Varuṇa and said, "O venerable one, teach me Brahman." To him, Varuṇa replied, "Seek to know Brahman through austerity; austerity is Brahman."