Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.2.11

13.2.2.11

pañcadaśa pañcadaśo evetareṣu pañcadaśo vai vajro vīryaṃ vajro vajreṇavaitadvīryeṇa yajamāno'bhitaḥ pāpmānamapahate

And fifteen (victims), indeed, are (bound) to each of the other (stakes);--for fifteenfold is the thunderbolt, and the thunderbolt means manly vigour: with that thunderbolt, manly vigour, the Sacrificer now repels evil on both sides (of the sacrifice).