Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.4.5

1.6.4.5

te devā abruvan | na vā imamanyatsomāddhinuyātsomamevāsmai sambharāmeti tasmai somaṃ samabharanneṣa vai somo rājā devānāmannaṃ yaccandramāḥ sa yatraiṣa etāṃ rātriṃ na purastānna paścāddadṛśe tadimaṃ lokamāgacati sa ihaivāpaścauṣadhīśca praviśati sa vai devānāṃ vasvannaṃ hyeṣāṃ tadyadeṣa etāṃ rātrimihāmā vasati tasmādamāvāsyā nāma

The gods said, 'Nothing but Soma will satiate him: let us prepare Soma for him!' They prepared Soma for him. Now this king Soma, the food of the gods, is no other than the moon. When he (the moon, masc.) is not seen that night either in the east or in the west, then he visits this world; and here he enters into the waters (f.) and plants (f.). He is indeed a treasure for the gods, he is their food. And since during that night he here dwells together (amâ vas), therefore that (night of new moon) is called amâvâsyâ (the dwelling together, or at home).