Brihadaranyaka-Upanishad: Fifth Adhyâya, Fourth Brahmana

1. This (heart) indeed is even that, it was indeed the true (Brahman). And whosoever knows this great glorious first-born as the true Brahman, he conquers these worlds, and conquered likewise may that (enemy) be! Yes, whosoever knows this great glorious first-born as the true Brahman; for Brahman is the true.