Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.4.6

1.2.4.6

sa japati | indrasya bāhurasi dakṣiṇa ityeṣa vai vīryavattamo ya indrasya bāhurdakṣiṇastasmādāhendrasya bāhurasi dakṣiṇa iti sahasrabhṛṣṭiḥ śatatejā iti sahasrabhṛṣṭirvai sa vajra āsīcatatejā yaṃ taṃ vṛtrāya prāharattamevaitatkaroti

He murmurs (Vâg. S. I, 24): 'Thou art Indra's right arm!' for Indra's right arm no doubt is the most powerful one, and for that reason he says: 'Thou art Indra's right arm!' 'The thousand-spiked, hundred-edged! he adds, for a thousand spikes and a hundred edges had that thunderbolt which he hurled at Vritra: he thereby makes it to be that (thunderbolt).