Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.3.7

5.2.3.7

atha śvo bhūte | agnīṣomīyamekādaśakapālam puroḍāśaṃ nirvapati tena yatheṣṭyaivaṃ yajata etena vā indro vṛtramahanneteno eva vyajayata yāsyeyaṃ vijitistāṃ tatho evaiṣa etena pāpmānaṃ dviṣantam bhrātṛvyaṃ hanti tatho eva vijayate vijite 'bhaye 'nāṣṭre sūyā iti tasmādagnīṣomīya ekādaśakapālaḥ puroḍāśo bhavati tasyotsṛṣṭo gaurdakṣiṇotsarjaṃ vā amuṃ candramasaṃ ghnanti paurṇamāsenāha ghnantyāmāvāsyenotsṛjanti tasmādutsṛṣṭo gaurdakṣiṇā

And on the following day he prepares a cake on eleven potsherds for Agni and Soma, and offers it in the same way as an (ordinary) ishti, for it was thereby Indra slew Vritra, and thereby he gained that universal conquest which now is his. And in like manner does this (king, the Sacrificer) thereby slay his wicked, hateful enemy, and in like manner does he gain the victory. 'May I be consecrated, when safety and security from evil-doers have been gained!' thus. he thinks: hence there is a cake on eleven potsherds for Agni and Soma. For this (offering) a bull set at liberty is the sacrificial fee; for yonder moon they slay while setting him at liberty: to wit, by the full-moon offering they slay him, and by the new-moon offering they set him at liberty;--therefore a bull set at liberty is the fee.