Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.5.8

4.3.5.8

yadveva pratiprasthātā pratinigṛhṇīte | dvidevatyānvai prāviśantsa yāneva prāviśaṃstebhya evaitannirmimīte 'thāpidadhāti rakṣobhyo hyabibhayurviṣṇa urugāyaiṣa te somastaṃ rakṣasva mā tvā dabhanniti yajño vai viṣṇustadyajñāyaivaitatparidadāti guptyā athāha saṃsthita eva mādhyandine savane purā tṛtīyasavanādehi yajamāneti

And again, why the Pratiprasthâtri draws the Soma. They entered into the Dvidevatyas; and from those which they entered he thereby draws them out. He then covers it --for they were afraid of the Rakshas--with 'O Vishnu, Far-strider, here is thy Soma, protect it lest they should injure it!' For Vishnu is the sacrifice: to the sacrifice he thus makes it over for protection. Now, after the completion of the midday Soma feast and before the evening feast he says, 'Come hither, Sacrificer!'