Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.4.21

1.6.4.21

taddhaike | mahendrāyeti kurvantīndro vā eṣa purā vṛtrasya vadhādatha vṛtraṃ hatvā yathā mahārājo vijigyāna evam mahendro 'bhavattasmānmahendrāyeti tadvindrāye=tyeva kuryādindro vā eṣa purā vṛtrasya vadhādindro vṛtraṃ jaghnivāṃstasmādvindrāyetyeva kuryāt

Now some people offer (the Sânnâyya) to (Indra under the name of) 'Mahendra' (the great Indra), arguing, 'Before the slaying of Vritra he was Indra, it is true; but after slaying Vritra he became Mahendra, even as (a râgan, or king, becomes) a Mahârâga after obtaining the victory: hence (the Sânnâyya should be offered) to Mahendra.' Let him, nevertheless, offer it to 'Indra;' for Indra he was before the slaying of Vritra, and Indra he is after slaying Vritra: therefore let him offer it to 'Indra.'