Taittirīya Upaniṣad 3.5.1

3.5.1

vijñānaṃ brahmeti vyajānāt /
vijñānādadhyeva khalvimāni bhūtāni jāyante /
vijñānena jātāni jīvanti /
vijñānaṃ prayantyabhisam̐viśantīti /
tadvijñāya /
punareva varuṇaṃ pitaramupasasāra /
adhīhi bhagavo brahmeti /
tam̐ hovāca /
tapasā brahma vijijñāsasva /
tapo brahmeti /
sa tapo 'tapyata /
sa tapastaptvā // 1//
iti pañcamo '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 bliss (ānanda). For truly, indeed, beings here are born from bliss, when born they live by bliss, on deceasing they enter into bliss.