Jaiminīya Upaniṣad Brāhmaṇa 3.19

Now here, he lifts up the thunderbolt with speech.
When he says, 'The soma purifies itself', or 'Turn toward [us]', that thunderbolt of speech is grasped by speech itself; it is released through the truth of speech.
Therefore one should accompany with 'om' alone.

The gods, full of sap, mounted upward to the heavenly world by this threefold [knowledge].
Fearing the following after of human beings, they compressed the threefold veda.

As they pressed it, there was just one syllable they could not crush: this one, namely om.

And that indeed possesses the essence. Possessing the essence, truly, becomes the threefold knowledge of one who knows thus.

Whatever victory one wins by means of the threefold knowledge possessing the essence, whatever prosperity one attains, that victory he wins, that prosperity he attains, who knows thus.

This syllable indeed is the support of the threefold knowledge. With om the hotṛ is supported, with om the adhvaryu, with om the udgātṛ.

This syllable indeed is the summit of the Vedas. Placing the sacrificer in this syllable, the priests carry him up to the heavenly world. Therefore one should recite the responding chant with om alone. / Fifth khaṇḍa in the fourth anuvāka. / The fourth anuvāka is concluded.