Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.1.12

2.2.1.12

atha yadagnaye śucaye nirvapati | vīryaṃ vai śuci yadā vā annena vardhate 'tha vīryaṃ tadannenaivainametadvardhayitvāthāsminnetadvīryaṃ śuci dadhāti tasmādagnaye śucaye

And the reason why he makes offering to Agni Suki, is that brightness means vigour. Now when it (the child) grows by means of food, then there is vigour; and hence, when he has made it grow by means of food, he thereby puts into it that vigour, that brightness. This is why (he offers) to Agni Suki.