Shatapatha Brahmana 1.7.1.21

1.7.1.21

so 'pidadhāti | viṣṇo havyaṃ rakṣeti yajño vai viṣṇustadyajñāyaivaitaddhaviḥ paridadāti guptyai tasmādāha viṣṇo havyaṃ rakṣeti

He covers it over, with the formula (Vâg. S. I, 4 e), 'O Vishnu, protect the oblation!' for Vishnu, indeed, is the sacrifice; hence he thereby makes over this sacrificial food to the sacrifice for protection: for this reason he says, 'O Vishnu, protect the oblation!'