Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.4.3

1.6.4.3

te devā abruvan | amā vai no 'dya vasurvasati yo naḥ prāvātsīditi tābhyāmetadyathā jñātibhyāṃ vā sakhibhyāṃ vā sahāgatābhyāṃ samānamodanam pacedajaṃ vā tadaha mānuṣaṃ havirdevānāmevamābhyāmetatsamānaṃ havirniravapannaindrāgnaṃ dvādaśakapālam puroḍāśaṃ tasmādaindrāgno dvādaśakapālaṃ puroḍāśo bhavati

The gods said, 'Our Vasu, who has gone to live away from us, is this day dwelling together (amâ vas, viz. with Agni);' and as one would cook a dish of rice or a goat in common for two relatives or friends who have come to stay with him,--for such-like is human (fare), as the sacrificial food (havis) is that of the gods,--in like manner they offered to those two together that sacrificial food, the rice-cake on twelve potsherds for Indra and Agni. This is the reason why there is a rice-cake on twelve potsherds for Indra and Agni.