Taittirīya Upaniṣad 3.2.1

3.2.1

annaṃ brahmeti vyajānāt /
annādadhyeva khalvimāni
bhutāni jāyante /
annena jātāni jīvanti /
annaṃ prayantyabhisam̐viśantīti /
tadvijñāya /
punareva varuṇaṃ pitaramupasasāra /
adhīhi bhagavo brahmeti /
tam̐ hovāca /
tapasā brahma vijijñāsasva /
tapo brahmeti /
sa tapo 'tapyata /
sa tapastaptvā // 1//
iti dvitīyo 'nuvākaḥ //

Having understood that, he again approached his father Varuṇa, and said: ‘Declare Brahma, Sir!’

Then he said to him: ‘Desire to understand Brahma by austerity. Brahma is austerity (*tapas*).’

He performed austerity. Having performed austerity, he understood that Brahma is breath (*prāṇa*). For truly, indeed, beings here are born from breath, when born they live by breath, on deceasing they enter into breath.