TAITTIRĪYA UPANISHAD · 3.2

अन्नं ब्रह्मेति व्यजानात् । अन्नाद्ध्येव खल्विमानि
भूतानि जायन्ते । अन्नेन जातानि जीवन्ति ।
अन्नं प्रयन्त्यभिसंविशन्तीति । तद्विज्ञाय ।
पुनरेव वरुणं पितरमुपससार ।
अधीहि भगवो ब्रह्मेति । तंहोवाच ।
तपसा ब्रह्म विजिज्ञासस्व । तपो ब्रह्मेति ।
स तपोऽतप्यत । स तपस्तप्त्वा ।

annaṃ brahmeti vyajānāt । annāddhyeva khalvimāni
bhūtāni jāyante । annena jātāni jīvanti ।
annaṃ prayantyabhisaṃviśantīti । tadvijñāya ।
punareva varuṇaṃ pitaramupasasāra ।
adhīhi bhagavo brahmeti । taṃhovāca ।
tapasā brahma vijijñāsasva । tapo brahmeti ।
sa tapo’tapyata । sa tapastaptvā ।

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

Sri Aurobindo · CWSA 18, p. 226