Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.4.9

2.5.4.9

atha māhendraścarurbhavati | indro vā eṣa purā vṛtrasya badhādatha vṛtraṃ hatvā yathā mahārājo vijigyāna evaṃ mahendro 'bhavattasmānmāhendraścarurbhavati mahāntamu caivainametatkhalu karoti vṛtrasya badhāya tasmādveva māhendraścarurbhavati

Then follows a rice-part for Mahendra. For before the slaying of Vritra he was indeed Indra; but after slaying Vritra he became Mahendra (the great Indra), even as (a king becomes) a mahârâga, after obtaining the victory: hence there is a rice-pap for Mahendra. And thereby indeed he renders him great (strong), for the slaying of Vritra: for this reason also there is a rice-pap for Mahendra.