Khandogya-Upanishad: Eighth Prapathaka, Fifth Khanda
1. What people call sacrifice (yagna), that is really abstinence (brahmakarya). For he who knows, obtains that (world of Brahman, which others obtain by sacrifice), by means of abstinence.
What people call sacrifice (ishta), that is really abstinence, for by abstinence, having searched (ishtva), he obtains the Self.
2. What people call sacrifice (sattrayana), that is really abstinence, for by abstinence he obtains from the Sat (the true), the safety (trana) of the Self.
What people call the vow of silence (mauna), that is really abstinence, for he who by abstinence has found out the Self, meditates (manute).
3. What people call fasting (anasakayana), that is really abstinence, for that Self does not perish (na nasyati), which we find out by abstinence.
What people call a hermit's life (aranyayana), that is really abstinence. Ara and Nya are two lakes in the world of Brahman, in the third heaven from hence; and there is the lake Airanimadiya, and the Asvattha tree, showering down Soma, and the city of Brahman (Hiranyagarbha) Aparagita, and the golden Prabhuvimita (the hall built by Prabhu, Brahman).
Now that world of Brahman belongs to those who find the lakes Ara and Nya in the world of Brahman by means of abstinence; for them there is freedom in all the worlds.