Taittiriya Upanishad 3.4.1

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 //

he knew mind as Brahman; for from mind, indeed, spring all these beings; having been born, they are sustained by mind; and they move towards and merge into mind. Having known that, he approached his father Varuṇa again and made the (formal) request, "O revered sir, teach me Brahman. To him he (Varuṇa) said: "Crave to know Brahman through concentration; concentration is Brahman. He practised concentration. He having practised concentration,