Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.1.8

2.2.1.8

athāgnaye śucaye nirvapati | vīryaṃ vai śuci yadvā asyaitadujjvalatyetadasya vīryaṃ śuci vīryamevāsminnetaddadhāti tadvetayaivāsmiṃstaddadhāti yadā hyevāsminnetāmāhutiṃ juhotyathāsyaitadvīryaṃ śucyujjvalati

He then makes offering to Agni Suki (the Bright). Now brightness means vigour, so that he thereby puts vigour into him. And this he puts into him by means of this (offering); for when he offers up that oblation in him (Agni), then that vigour, that brightness of his blazes up.