Taittiriyaka-Upanishad: Third Valli, Fourth Anuvaka

He perceived that mind (manas) is Brahman, for from mind these beings are born; by mind, when born, they live; into mind 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—