Shatapatha Brahmana 13.5.1.17

13.5.1.17

yadakrandaḥ prathamaṃ jāyamāna iti triḥ prathamayā triruttamayā tāḥ pañcadaśa sampadyante pañcadaśo vai vajro vīryam vajro vajreṇaivaitadvīryeṇa yajamānaḥ purastātpāpmām vajro vajreṇaivaitadvīryeṇa yajamānaḥ purastātpāpmānamapahate tadvai yajamānāyaiva vajraḥ pradīyate yo'sya stṛtyastaṃ startava upa prāgācasanam vājyarvopa prāgātparamaṃyatsadhasthamiti

'When, first born, thou didst neigh...'--thrice (he praises) with the first, and thrice with the last (verse), these amount to fifteen,--fifteenfold is the thunderbolt, and the thunderbolt means vigour: with that thunderbolt, vigour, the Sacrificer thus from the very first repels evil: thus, indeed, it is to the Sacrificer that the thunderbolt is given in order to smite for him whoever is to be smitten.--[Rig-veda I, 163, 12. 13], The swift racer hath gone forward to the slaying....'--'The racer hath gone forward to the highest place...'--