Chandogya Upanishad
Prapathaka II, Khanda 10
Next let a man meditate on the sevenfold Sâman which is uniform in itself and leads beyond death. The word hiṅkâra has three syllables, the word prastâva has three syllables: that is equal (sama).
The word âdi (first, Om) has two syllables, the word pratihâra has four syllables. Taking one syllable from that over, that is equal (sama).
The word udgîtha has three syllables, the word upadrava has four syllables. With three and three syllables it should be equal. One syllable being left over, it becomes trisyllabic. Hence it is equal.
With twenty-one syllables a man reaches the sun (and death), for the sun is the twenty-first from here; with the twenty-second he conquers what is beyond the sun: that is blessedness, that is freedom from grief