Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.3.8

1.1.3.8

yuṣmā indro 'vṛṇīta vṛtratūrya iti | etā u hīndro 'vṛṇīta vṛtreṇa spardhamāna etābhirhyenamahaṃstasmādāha yuṣmā indro 'vṛṇīta vṛtratūrya iti

And further (Vâg. S. I, 13 a): 'You Indra chose (for his companions) in the battle against Vritra!' For Indra, when he was battling with Vritra, did choose them (the waters) and with their help he killed him; and for this reason he says, 'You Indra chose in the battle against Vritra!'