Taittirīyopaniṣat 03.04

स तपोऽतप्यत। स तपस्तप्त्वा। मनो ब्रह्मेति व्यजानात्। मनसो ह्येव खल्विमानि भूतानि जायन्ते। मनसा जातानि जीवन्ति। मनः प्रयन्त्यभिसंविशन्तीति। तद्विज्ञाय। पुनरेव वरुणं पितरमुपससार। अधीहि भगवो ब्रह्मेति। तगं होवाच। तपसा ब्रह्म विजिज्ञासस्व। तपो ब्रह्मेति ॥४॥

He practiced austerity. After performing austerity, he realized that "the mind is Brahman." Indeed, from the mind alone all these beings are born; by the mind they live, and into the mind they dissolve. Having understood this, he again approached his father Varuṇa and said, "O venerable one, teach me about Brahman." Varuṇa replied, "Seek to know Brahman through austerity, for austerity is Brahman."