Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.3.17

4.3.3.17

yadveva māhendraṃ grahaṃ gṛhṇāti | indro vā eṣa purā vṛtrasya badhādatha vṛtraṃ hatvā yathā mahārājo vijigyāna evam mahendro 'bhavattasmānmāhendraṃ grahaṃ gṛhṇāti mahāntamu caivainametatkhalu karoti vṛtrasya badhāya tasmādveva māhendraṃ grahaṃ gṛhṇāti śu pātreṇa gṛhṇātyeṣa vai śukro ya eṣa tapatyeṣa u eva mahāṃstasmācukrapātreṇa gṛhṇāti

And again, why he draws the Mâhendra cup. Before the slaughter of Vritra, he was indeed Indra; but when he had slain Vritra, he became the Great--Indra, even as one who has conquered all around, becomes a Great King (mahârâga): therefore he draws the Mâhendra cup. And, moreover, he forsooth make's him great for the slaughter of Vritra therefore also he draws the Mâhendra cup. He draws it in the Sukra vessel, for bright (sukra) and great indeed is he (the sun) that burns yonder: therefore he draws it in the Sukra vessel.