Rigvidhana 1.5.22

1.5.22

ante dvādaśa rātrasya sthālī pākam prakalpayet./
agnaye ca atha somāya tṛtīyām ca tayoḥ saha.//

At the end of the twelve nights, he should prepare a cooked mess of grain; / [offering] to Agni, and then to Soma, and a third to both of them together,