TAITTIRĪYA UPANISHAD · 3.3

प्राणो ब्रह्मेति व्यजानात् । प्राणाद्ध्येव खल्विमानि
भूतानि जायन्ते । प्राणेन जातानि जीवन्ति ।
प्राणं प्रयन्त्यभिसंविशन्तीति । तद्विज्ञाय ।
पुनरेव वरुणं पितरमुपससार ।
अधीहि भगवो ब्रह्मेति । तंहोवाच ।
तपसा ब्रह्म विजिज्ञासस्व । तपो ब्रह्मेति ।
स तपोऽतप्यत । स तपस्तप्त्वा ।

prāṇo brahmeti vyajānāt । prāṇāddhyeva khalvimāni
bhūtāni jāyante । prāṇena jātāni jīvanti ।
prāṇaṃ prayantyabhisaṃviśantīti । tadvijñāya ।
punareva varuṇaṃ pitaramupasasāra ।
adhīhi bhagavo brahmeti । taṃhovāca ।
tapasā brahma vijijñāsasva । tapo brahmeti ।
sa tapo’tapyata । sa tapastaptvā ।

He knew Prana for the Eternal. For from Prana alone, it appeareth, are these creatures born and being born they live by Prana and to Prana they go hence and return. And when he had known this, he came again to Varouna his father and said “Lord, teach me the Eternal.” But his father said to him “By askesis do thou seek to know the Eternal, for askesis in thought is the Eternal.” He concentrated himself in thought and by the energy of his brooding

Sri Aurobindo · CWSA 18, p. 226