Chandogya Upanishad
The teachers of Brahman (Veda) declare, as the Prâtah-savana (morning-oblation) belongs to the Vasus, the Mâdhyandina-savana (noon-libation) to the Rudras, the third Savana (evening-libation) to the Âdityas and the Visve Devas,
Where then is the world of the sacrificer? He who does not know this, how can he perform the sacrifice? He only who knows, should perform it.
Before the beginning of the Prâtaranuvâka (matin-chant), the sacrificer, sitting down behind the household altar (gârhapatya), and looking towards the north, sings the Sâman, addressed to the Vasus:
'Open the door of the world (the earth), let us see thee, that we may rule (on earth).'
Then he sacrifices, saying: 'Adoration to Agni, who dwells on the earth, who dwells in the world! Obtain that world for me, the sacrificer! That is the world for the sacrificer!'
'I (the sacrificer) shall go thither, when this life is over. Take this! (he says, in offering the libation.) Cast back the bolt!' Having said this, he rises. For him the Vasus fulfil the morning-oblation.
Before the beginning of the Mâdhyandinasavana, the noon-oblation, the sacrificer, sitting down behind the Âgnidhrîya altar, and looking towards the north, sings the Sâman, addressed to the Rudras:
'Open the door of the world (the sky), let us see thee, that we may rule wide (in the sky).'
Then he sacrifices, saying: 'Adoration to Vâyu (air), who dwells in the sky, who dwells in the world. Obtain that world for me, the sacrificer! That is the world Tor the sacrificer!'
'I (the sacrificer) shall go thither, when this life is over. Take this! Cast back the bolt!' Having said this, he rises. For him the Rudras fulfil the noon-oblation.
Before the beginning of the third oblation, the sacrificer, sitting down behind the Âhavanîya altar, and looking towards the north, sings the Sâman, addressed to the Âdityas and Visve Devas:
'Open the door of the world (the heaven), let us see thee, that we may rule supreme (in heaven).' This is addressed to the Âdityas.
Next the Sâman addressed to the Visve Devas: 'Open the door of the world (heaven), let us see thee, that we may rule supreme (in heaven).'
Then he sacrifices, saying: 'Adoration to the Âdityas and to the Visve Devas, who dwell in heaven, who dwell in the world. Obtain that world for me, the sacrificer!'
'That is the world for the sacrificer! I (the sacrificer) shall go thither, when this life is over. Take this! Cast back the bolt!' Having said this, he rises.
For him the Âdityas and the Visve Devas fulfil the third oblation. He who knows this, knows the full measure of the sacrifice, yea, he knows it.