Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.5.3

5.2.5.3

atha yadaindrāvaiṣṇavaḥ | carurbhavatīndro vai yajamāno vaiṣṇavāḥ puruṣāstadasmā agnirdātā puruṣāndadāti tairevaitatsaṃspṛśate tānātmankurute

And as to why there is a pap for Indra and Vishnu;--Indra is the Sacrificer, and men are Vishnu's: thus Agni, the giver, gives him (the Sacrificer) men; he now puts himself in contact with them, makes them his own.