Aitareyabrahmana 2.8

2.8

The gods slaughtered man as a victim. From him, when slaughtered, the sacrificial essence went out;
it entered the horse; therefore the horse became fit for sacrifice.
Then they dismissed him whose sacrificial essence had gone out; he became a kimpuruṣa.
They slaughtered the horse. From the horse, when slaughtered, it went out; it entered the cow;
therefore the cow became fit for sacrifice. Then they dismissed it whose sacrificial essence had gone out;
it became a wild ox.
They slaughtered the cow. From the cow, when slaughtered, it went out; it entered the sheep;
therefore the sheep became fit for sacrifice. Then they dismissed it whose sacrificial essence had gone out;
it became a gayal. They slaughtered the sheep. From the sheep, when slaughtered, it went out;
it entered the goat; therefore the goat became fit for sacrifice.
Then they dismissed it whose sacrificial essence had gone out; it became a camel.
It rested longest, as it were, in the goat;
therefore of these animals the goat is the most commonly employed.
They slaughtered the goat. From the goat, when slaughtered, it went out; it entered this [earth];
therefore this [earth] became fit for sacrifice. Then they dismissed it whose sacrificial essence had gone out;
it became a śarabha.
These are the animals whose sacrificial essence has departed; they are unfit for sacrifice; therefore one should not eat of them.
They followed it into this [earth]; followed after, it became rice.
Hence they offer a sacrificial cake after the victim, saying: 'May our sacrifice be performed with a victim possessing the sacrificial essence;
may our sacrifice be performed with a whole victim.'
With a victim possessing the sacrificial essence his sacrifice is performed,
with a whole victim his sacrifice is performed, who knows this.