Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.3.7

1.7.3.7

so 'dhvaryuḥ | yathāpūrvaṃ havīṃṣyabhyaghārayadekasmā avadānāya punarāpyāyayadayātayāmānyakarottata ekaikamavadānamavādyattasmādvāstavya ityāhurvāstu hi tadyajñasya yaddhuteṣu haviḥṣu tasmādyasyai kasyai ca devatāyai havirgṛhyate sarvatraiva sviṣṭakṛdanvābhaktaḥ sarvatra hyevainaṃ devā anvābhajan

The Adhvaryu accordingly sprinkled the sacrificial dishes in proper succession, and replenished them for the sake of one (additional) portion, and again rendered them fit for use, and cut off one portion for each. This then is the reason why he (Rudra) is called Vâstavya, for a remainder (vâstu) is that part of the sacrifice which (is left) after the oblations have been made: hence, if sacrificial food is offered to any deity, the Svishtakrit (Agni, 'the maker of good offering') is afterwards invariably offered a share of it; because the gods invariably gave him a share after themselves.