Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.2.41

7.5.2.41

tadyāḥ pañcadaśa pūrvāḥ | tā apasyā vajro vā āpo vajraḥ pañcadaśastasmāṃdyenāpo yantyapaiva tatra pāpmānaṃ ghnanti vajro haiva tasyārdhasya pāpmānamapahanti tasmādvarṣatyaprāvṛto vrajedayam me vajraḥ pāpmānamapahanaditi

Now the first fifteen are the Apasyâh,--water is a thunderbolt, and the thunderbolt is fifteenfold;--hence wherever the waters flow, there they destroy evil; and verily the thunderbolt destroys the evil of this place: hence, when it rains one should go about uncovered, thinking, 'May that thunderbolt remove evil from me!'