Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.4.17

2.4.4.17

atha yatpūrvedyuḥ | aindrāgnena yajate 'māvāsyāyāṃ darśapūrṇamāsayorvai devate sta indrāgnī eva te evaitadañjasā pratyakṣaṃ yajatyañjasā ha vā asya darśapūrṇamāsābhyāmiṣṭam bhavati ya evametadveda

And again as to why, at new moon, he offers the Indra-Agni (cake) on the first day. Indra and Agni doubtless are the deities of the new and full moon: it is to these, therefore, that he offers directly and expressly; and directly to the new and full moon is offering made by him who thus knows this.