Taittirīyopaniṣat 03.02

स तपोऽतप्यत। स तपस्तप्त्वा। अन्नं ब्रह्मेति व्यजानात्। अन्नाद्ध्येव खल्विमानि भूतानि जायन्ते। अन्नेन जातानि जीवन्ति। अन्नं प्रयन्त्यभिसंविशन्तीति। तद्विज्ञाय। पुनरेव वरुणं पितरमुपससार। अधीहि भगवो ब्रह्मेति। तगं होवाच। तपसा ब्रह्म विजिज्ञासस्व। तपो ब्रह्मेति ॥२॥

He practiced austerity. After performing austerity, he realized, "Food is Brahman." Indeed, it is from food that all these beings are born; by food, they live; and into food, they finally merge. Understanding this, he again approached his father Varuṇa and said, "O revered one, teach me about Brahman." Varuṇa replied, "Seek to know Brahman through austerity; austerity itself is Brahman."