Khandogya-Upanishad: Second Prapathaka, Twenty-Third Khanda

2. Austerity the second, and to dwell as a Brahmakarin in the house of a tutor, always mortifying the body in the house of a tutor, is the third. All these obtain the worlds of the blessed; but the Brahmasamstha alone (he who is firmly grounded in Brahman) obtains immortality.

3. Pragâpati brooded on the worlds. From them, thus brooded on, the threefold knowledge (sacrifice) issued forth. He brooded on it, and from it, thus brooded on, issued the three syllables, Bhuh, Bhuvah, Svah.

4. He brooded on them, and from them, thus brooded on, issued the Om. As all leaves are attached to a stalk, so is all speech (all words) attached to the Om (Brahman). Om is all this, yea, Om is all this.