Shatapatha Brahmana 11.1.4.4

11.1.4.4

tadyadevam bhavati eṣa vai somo rājā devānāmannaṃ yaccandramāstametadupaitsīttamapārātsīttamasmā agnirdātā dadātīndraḥ pradātā prayacati tamasmā indrāgnī yajñaṃ dattastenendrāgnibhyāṃ dattena yajñena yajate'tha yadviṣṇave śipiviṣṭāyeti yajño vai viṣṇuratha yacipiviṣṭāyeti yamupaitsīttamapārātsīttacipitamiva yajñasya bhavati tasmācipiviṣṭāyeti tatro yacaknuyāttaddadyānnādakṣiṇaṃ haviḥ syāditi hyāhuratha yadaiva nodiyādathopavaset

And as to this being so, it is because that moon is no other than King Soma, the food of the gods;--he (the Sacrificer) at that time sought to secure him, and missed him: Agni, the Giver, gives that (moon) to him, and Indra, the Bestower, bestows that one upon him; Indra and Agni give that (moon, Soma) as a sacrifice to him, and that sacrifice given by Indra and Agni he offers. And as to why (he offers) to Vishnu, the Bald, it is because Vishnu is the sacrifice; and as to why to the Bald (sipivishta),--it is that his missing him whom he sought to secure is the bald part (? sipita) of the sacrifice, hence to the Bald one. And on this occasion he should give (to the priests) as much as he is able to give, for no oblation, they say, should be without a dakshinâ. And let him observe the fast just (on the day) when he (the moon) does not rise.