Shatapatha Brahmana 6.2.1.15

6.2.1.15

puruṣo 'śvo gauravirajo bhavanti | etāvanto vai sarve paśavo 'nnam paśavastadyāvadannaṃ tadetatpurastānnidadhāti tadenam paśyannagnirupāvartate

There are a man, a horse, a bull, a ram, and a he-goat; for such are all the animals (used for sacrifice). Animals are food: he thus lays down in front whatever food there is; and seeing that, Agni turns unto him.