Chandogya Upanishad
The udgîtha, of which a poet said, I choose the deep sounding note of the Sâman as good for cattle, belongs to Agni; the indefinite note belongs to Pragâpati, the definite note to Soma, the soft and smooth note to Vâyu, the smooth and strong note to Indra, the heron-like note to Brihaspati, the dull note to Varuna. Let a man cultivate all of these, avoiding, however, that of Varuna.
Let a man sing, wishing to obtain by his song immortality for the Devas. 'May I obtain by my song oblations (svadhâ) for the fathers, hope for men, fodder and water for animals, heaven for the sacrificer, food for myself,' thus reflecting on these in his mind, let a man (Udgâtri priest) sing praises, without making mistakes in pronunciation, &c.
All vowels (svara) belong to Indra, all sibilants (ûshman) to Pragâpati, all consonants (sparsa) to Mrityu (death). If somebody should reprove him for his vowels, let him say, 'I went to Indra as my refuge (when pronouncing my vowels): he will answer thee.'
And if somebody should reprove him for his sibilants, let him say, 'I went to Pragâpati as my refuge: he will smash thee.' And if somebody should reprove him for his consonants, let him say, 'I went to Mrityu as my refuge: he will reduce thee to ashes.'
All vowels are to be pronounced with voice (ghosha) and strength (bala), so that the Udgâtri may give strength to Indra. All sibilants are to be pronounced, neither as if swallowed (agrasta), nor as if thrown out (nirasta), but well opened (vivrita), so that the Udgâtri may give himself to Pragâpati. All consonants are to be pronounced slowly, and without crowding them together, so that the Udgâtri may withdraw himself from Mrityu.