Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.1.26

2.6.1.26

athāha pitṛbhyaḥ somavadbhyo 'nubrūhīti | somāya vā pitṛmate sa dve puro 'nuvākye anvāhaikayā vai devānpracyāvayanti dvābhyām pitṝntsakṛdu hyeva parāñcaḥ pitarastasmāddve puro 'nuvākye anvāha

[The Adhvaryu] then says, 'Pronounce the invitatory prayer to the fathers, accompanied by Soma!' or '--to Soma, accompanied by the fathers'!--Two invitatory prayers he (the Hotri) pronounces (at the offerings), because it is with one that one moves the gods, and with two the fathers, since the fathers have departed once (for all): hence he pronounces two invitatory prayers.