Verse 2.4.4.15

atha yatpūrvedyuḥ | agnīṣomīyeṇa yajate paurṇamāsyāmetena vā indro vṛtramahanneteno eva vyajayata yāsyeyaṃ vijitistāṃ tatho evaiṣa etena pāpmānaṃ dviṣantam bhrātṛvyaṃ hanti tatho eva vijayate'tha yatsaṃnayatyāmāvāsyaṃ vai sāṃnāyyaṃ dūre tadyadamāvāsyeti kṣirra evaitadvṛtraṃ jaghnuṣe tametena rasenāprīṇan kṣipre ha vai pāpmānamapahate ya evaṃ vidvānpaurṇamāsyāṃ saṃnayatyeṣa vai somo rājā d s02401150Levānāmannaṃ yaccandramāstametatpūrvedyurabhiṣuṇvanti prātarbhakṣayiṣyantastametadbhakṣayanti yadapakṣīyate

Preview of English translation:

15. And again when, at full moon, he offers the Agni-Soma (cake) on the first day,—it was by that (offering) that Indra slew Vritra; it was thereby he gained that supreme authority which he now wields: and so does he (the sacrificer) thereby slay his wicked spiteful enemy and gain the superiority. And as to his mixing (sweet and sour milk),—the Sannayya is (the oblation) of the new moon (ama-vasya), and the new moon means being far away: to him who had slain Vritra this was forthwith (offered), and him they regaled with that draught. He therefore who, knowing this, prepares the Sannayya at full moon, forthwith drives away evil. Now that moon doubtless is king Soma, the food of the gods: they extract it on the first day, intending to consume it on the next day; consequently when that (moon) wanes, it is being consumed by them.

For a detailled translation, including proper diacritics and footnotes, go the full English translation.