Shatapatha Brahmana 11.1.5.8

11.1.5.8

atha yadagnaye vaiśvānarāya dvādaśakapālam puroḍāśaṃ nirvapati yatra vā indro vṛtramahaṃstamagninā vaiśvānareṇa samadahattadasya sarvam pāpmānaṃ samadahattatho evaiṣa etadindreṇaiva vṛtraghnā pāpmānaṃ vṛtraṃ hatvā tamagninā vaiśvānareṇa saṃdahati tadasya sarvam pāpmānaṃ saṃdahati sa yo haivaṃ vidvānetayeṣṭyā yajate na hāsyālpaścana pāpmā pariśiṣyate

And as to why he prepares a cake on twelve potsherds for Agni Vaisvânara,--when Indra had slain Vritra, he burnt him completely by means of Agni Vaisvânara, and thereby burnt all his (Vritra's) sin; and in like manner does that (Sacrificer) now, after slaying sin, Vritra, with the help of Indra Vritrahan, burn him, and all that sin of his, by means of Agni Vaisvânara; and, verily, not the slightest sin remains in him who, knowing this, performs this offering.