Shatapatha Brahmana 1.6.1.9

1.6.1.9

tadāhuḥ | yaduttamānprayājānāvāhayantyatha kasmādenānprathamānyajantītyuttamānhyenānyajñe 'vākalpayanprathamānvo yajāmetyabruvaṃstasmāduttamānāvāhayanti prathamānyajanti

Now, as to this point, some raise the objection, 'But since they invite the fore-offerings last of all (at the two libations of butter), why do they offer prayer to them first of all?'--Because they established them last of all in the sacrifice; and because they said, 'we will offer prayer to you first:' for that reason they invite them last, and offer prayer to them first.