Taittiriyaka-Upanishad: Third Valli, Fifth Anuvaka

He perceived that understanding (vignana) was Brahman, for from understanding these beings are born; by understanding, when born, they live; into understanding 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—