Shatapatha Brahmana 11.2.5.3

11.2.5.3

tadāhuḥ vivṛtte'śvasyeṣṭiṃ nirvapatītyeṣa vai somo rājā devānāmannaṃ yaccandramāḥ sa yatraiṣa etāṃ rātriṃ na purastānna paścāddadṛśe tadimaṃ lokamāgacati so'smiṃloke vivartate

As to this, they say, 'On the starting off of the horse he performs an offering; for the moon, doubtless, is the same as King Soma, the food of the gods: when, during that night (of new moon), he does not appear either in the east or in the west, then he comes to this world, and starts for this world.'