Speech is the Rig Veda, breath is the Samaveda, the syllable OM is the Udgitha. Now, these [two] form a union, speech with breath and the Rig Veda with the Samaveda.
Their union consists in OM; it is in this syllable that they are united. Whenever they come together and unite, they fulfill each other's desire.
Sama Veda, Chandogya Upanishad I, 1, 6