Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.4.11

2.4.4.11

atha yatpūrvedyuḥ | agnīṣomīyeṇa yajate paurṇamāsyāṃ yamevāmumupavasathe 'gnīṣomīyam paśumālabhate sa evāsya saḥ

And when, at full moon, he offers the Agni-Soma (cake) on the first day, then this is for him that victim which they slaughter for Agni and Soma on the fast-day (of the Soma-sacrifice).