Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 3.7.14

3.7.14

atha yad agnim prathamam devatānām yajati /

iii. 6. In that he offers to Agni first of the deities, and because Agni is the mouth (beginning) of the gods, verily thus at the beginning he delights the gods. Now as to his sacrificing to Agni and Soma at the full moon offering, Agni and Soma were within Vṛtra; with regard to them Indra could not hurl his bolt; for them he arranged the portion at the full moon