The Upanishads, Part 2 (SBE15), by Max Müller, , at sacred-texts.com
He perceived that breath 1 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--
65:1 Or life; see Brih. Âr. Up. IV, 1, 3.