Shatapatha Brahmana 13.1.1.1

13.1.1.1

brahmaudanam pacati reta eva taddhatte yadājyamuciṣyate tena raśanāmabhyajyādatte tejo vā ājyam prājāpatyo'śvaḥ prajāpatimeva tejasā samardhayatyapūto vā eṣo'medhyo yadaśvaḥ

He (the Adhvaryu) cooks the priests’ mess of rice: it is seed he thereby produces. Having greased a rope with the ghee which is left over, he takes it; for ghee is (a type of) fiery spirit, and the horse is sacred to Pragâpati: he thus endows Pragâpati with fiery spirit. Impure, and unfit for sacrifice, indeed, is that (animal), to wit, the horse.