Taittiriya Upanishad 3.4

3.4.1

mano brahmeti vyajānāt /
manaso hyeva khalvimāni
bhūtāni jāyante /
manasā jātāni jīvanti /
manaḥ prayantyabhisam̐viśantīti /
tadvijñāya /
punareva varuṇaṃ pitaramupasasāra /
adhīhi bhagavo brahmeti /
tam̐ hovāca /
tapasā brahma vijijñāsasva /
tapo brahmeti /
sa tapo 'tapyata /
sa tapastaptvā // 1//
iti caturtho '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.'

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