Taittiriyaka-Upanishad: Third Valli, Third Anuvaka

He perceived that breath is Brahman, for from breath these beings are born; by breath, when born, they live; into breath they enter at their death.

Having perceived this, he went again to his father Varuna, saying: “Sir, teach me Brahman.” He said to him: “Try to know Brahman by penance, for penance is (the means of knowing) Brahman.”

He performed penance. Having performed penance—