Shatapatha Brahmana 11.1.5.7

11.1.5.7

atha yadindrāya vṛtraghne pāpmā vai vṛtro yo bhūtervārayitvā tiṣṭhati kalyāṇātkarmaṇaḥ sādhostametadindreṇaiva vṛtraghnā pāpmānaṃ vṛtraṃ hanti tasmādindrāya vṛtraghne

And as to why to Indra Vritrahan,--Vritra is sin: with the help of Indra, the slayer of Vritra, he thus slays sin, Vritra, which ever keeps him from well-being, from virtue, and from the good work: this is why he (offers) to Indra Vritrahan.