Chandogya Upanishad
Prapathaka I, Khanda 13
The syllable Hâu is this world (the earth), the syllable Hâi the air, the syllable Atha the moon, the syllable Iha the self, the syllable Î is Agni, fire.
The syllable Û is the sun, the syllable E is the Nihava or invocation, the syllable Auhoi is the Visve Devas, the syllable Hiṅ is Pragâpati, Svara (tone) is breath (prâna), the syllable Yâ is food, the syllable Vâg is Virâg.
The thirteenth stobha syllable, viz. the indistinct syllable Huṅ, is the Undefinable (the Highest Brahman).