Chandogya Upanishad
Prapathaka II, Khanda 21
The hiṅkâra is the threefold knowledge, the prastâva these three worlds, the udgîtha Agni (fire), Vâyu (air), and Âditya (sun), the pratihâra the stars, the birds, and the rays, the nidhana the serpents, Gandharvas, and fathers. That is the Sâman, as interwoven in everything.
And thus it is said in the following verse: There are the fivefold three (the three kinds of sacrificial knowledge, the three worlds &c. in their fivefold form, i. e. as identified with the hiṅkâra, the prastâva, &c.), and the other forms of the Sâman. Greater than these there is nothing else besides.'