Śatapatha Brāhmaṇa ("of a hundred paths") 5.4.3.18

5.4.3.18

indrasyendriyāya svāheti | sa yadevaindraṃ rathasya tadevaitena prīṇāti savyaṣṭhā vā aindro rathasya savyaṣṭhāramevaitena prīṇātīndriyaṃ vai vīryamindra indriyamevāsyaitadvīryaṃ rājyamabhivimucyate

'To Indra's energy, hail!' He thereby pleases the part of the chariot that belongs to Indra. Now the warrior relates to Indra, and it is the warrior he thereby pleases. And Indra's energy (indriya) means the vigour in Indra: it is in regard to energy, vigour, that his kingship is thereby rendered free.