Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.19

6.2.2.19

sa vā iṣṭvaiva paurṇamāsena | atha paśumālabheta paurṇamāsena vā indro vṛtram pāpmānaṃ hatvāpahatapāpmaitatkarmārabhata tathaivaitadyajamānaḥ paurṇamāsenaiva vṛtram pāpmānaṃ hatvāpahatapāpmaitatkarmārabhate

Now, as soon as he has performed the full-moon offering, let him slaughter the victim. For Indra, having driven away Vritra, evil, by means of the full-moon offering, thus freed from evil entered upon this sacrificial performance; and in like manner the Sacrificer, having driven away Vritra, evil, by means of the full-moon offering, thus freed from evil now enters on this (sacred) performance.