Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.1.48

2.6.1.48

taddhaike | haviruciṣṭamanusamasyanti tadu tathā na kuryāddhutociṣṭaṃ vā etanneddhutociṣṭamagnau juhavāmeti tasmādapo vaivābhyavahareyuḥ prāśnīyurvā

And here some throw also the remaining sacrificial food into the fire; but let him not do so; for that (remaining havis) is the residue of an offering; and lest he should offer the residue of an offering, let them (the priests) rather throw it into the water or eat it.