Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.1.23

6.3.1.23

te 'bruvan | yadyaha sarvairanveṣiṣyāmo yātayāmā anupajīvanurvā gave 'śvo vāśvāya puruṣo vā puruṣāya īyā bhaviṣyanti yadyu asarvairasarvamanuvetsyāma iti ta etamekam paśuṃ dvābhyām paśubhyām pratyapaśyanrāsabhaṃ goścāveśca tadyadetamekam paśuṃ dvābhyām paśubhyām pratyapaśyaṃstasmādeṣa ekaḥ sandviretāḥ

They said, 'Surely, if we search with all of them, they will become used up and affording no livelihood; and if not with all, we shall get him (Agni) incomplete.' They saw one animal (as a substitute) for two animals, namely, the ass (as a substitute) for the cow and the sheep; and because they saw that one beast (would do) for two beasts, therefore that one (the he-ass), whilst being one, doubly impregnates.