Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.5.24

4.3.5.24

yadveva pratiprasthātā saṃsravau sampragṛhṇāti | purā vā ebhya etanmiśrādgrahamahauṣuḥ purā tṛtīyasavanāttṛtīyasavanāya vā eṣa graho gṛhyate tadādityāstṛtīyasavanamapiyanti tathā na bahirdhā yajñādbhavanti tasmātpratiprasthātā saṃsravau sampragṛhṇāti

And again why the Pratiprasthâtri holds out the two residues. Previous to that mixed (press-feast), previous to the evening feast, they have offered that (unmixed or special) libation to those (Âdityas); but this libation is taken for the evening feast: thereby the Âdityas take part in the evening feast, and thus they are not excluded from the sacrifice. This is why the Pratiprasthâtri holds out the two residues.