Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 3.5.15

3.5.15

atha yat paurṇamāsyām vārtraghnāv ājya bhāgau bhavataḥ /

iii. 5. In that the butter portions at the full moon offering contain references to the slaying of Vṛtra, it is because Indra slew Vṛtra with the full moon offering. In that they have references to growth at the new moon offering, it is because then the moon becomes destroyed and verily thus does he cause it to swell up and to increase The offering verses in these offerings contain the word 'rejoicing', for they include the same oblations as the fore-offerings. Now 'rejoicing' is the holy power; verily thus by the holy power he offers the oblation to the gods. These offerings are threefold, *ye yajāmahe*, the Nigada, the sound *vaṣaṭ*; the butter portions