Taittiriyaka-Upanishad: Third Valli, Second Anuvaka

He perceived that food is Brahman, for from food these beings are produced; by food, when born, they live; and into food 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—