Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.1.18

6.2.1.18

puruṣam prathamamālabhate | puruṣo hi prathamaḥ paśūnāmathāśvam puruṣaṃ hyanvaśvo 'tha gāmaśvaṃ hyanu gaurathāviṃ gāṃ hyanvavirathājamaviṃ hyanvajastadenānyathāpūrvaṃ yathāśreṣṭhamālabhate

A man (purusha) he slaughters first, for man is the first of animals; then a horse, for the horse comes after man; then a bull, for the bull (or cow) comes after the horse; then a ram, for the sheep comes after the cow; then a he-goat, for the goat comes after the sheep: thus he slaughters them according to their form, according to their excellence.