Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.5.4

5.2.5.4

atha yadvaiṣṇavaḥ | trikapālo vā puroḍāśo bhavati carurvā yānevāsmā agnirdātā puruṣāndadāti teṣvevaitadantataḥ pratitiṣṭhati yadvai puruṣavānkarma cikīrṣati śaknoti vai tatkartuṃ tatpuruṣānevaitadupaiti puruṣavāntsūyā iti tasya vāmano gaurdakṣiṇā sa hi vaiṣṇavo yadvāmanaḥ

And as to why there is either a cake on three potsherds, or a pap, for Vishnu;--whatever men Agni, the giver, gives him, among them he thereby finally establishes him (the king); and whatever work he wishes to do with his men, that he is able to do. Thus he thereby approaches the men, thinking, 'May I be consecrated, and possessed of men!' A dwarfish bull is the sacrificial fee for this (offering), for the dwarf belongs to Vishnu.