Brihadaranyaka-Upanishad: Fifth Adhyâya, First Brahmana
1. That (the invisible Brahman) is full, this (the visible Brahman) is full. This full (visible Brahman) proceeds from that full (invisible Brahman). On grasping the fulness of this full (visible Brahman) there is left that full (invisible Brahman).
Om (is) ether, (is) Brahman. “There is the old ether (the invisible), and the (visible) ether of the atmosphere,” thus said Kauravyayaniputra. This (the Om) is the Veda (the means of knowledge), thus the Brahmanas know. One knows through it all that has to be known.