Chandogya Upanishad 5.4

'The altar (on which the sacrifice is supposed to be offered) is that world (heaven), O Gautama; its fuel is the sun itself, the smoke his rays, the light the day, the coals the moon, the sparks the stars.

'On that altar the Devas (or prânas, represented by Agni, &c.) offer the sraddhâ libation (consisting of water). From that oblation rises Soma, the king (the moon).