Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.4.1

1.2.4.1

indro ha yatra vṛtrāya vajram prajahāra | sa prahṛtaścaturdhā 'bhavattasya sphyastṛtīyaṃ vā yāvadvā yūpastṛtīyaṃ vāyāvadvā rathastṛtīyaṃ vā yāvadvātha yatra prāharattacakalo 'śīryata sa patitvā śarā 'bhavattasmācaro nāma yadarśāryataivamu sa caturdhā vajro 'bhavat

When Indra hurled the thunderbolt at Vritra, that hurled one became fourfold. Of (three parts of) it the wooden sword (sphya) represents one-third or thereabouts, the sacrificial post one-third or thereabouts, and the chariot one-third or thereabouts. That piece, moreover, with which he struck him, was broken off (sri); and on falling down it became an arrow (sara): hence the designation arrow, because it was broken off. And in this way the thunderbolt became fourfold.