Shatapatha Brahmana 2.3.2.13

2.3.2.13

atha yatraitadaṅgārāścākaśyanta iva | tarhi haiṣa bhavati brahma sa yaḥ kāmayeta brahmavarcasī syāmiti tarhi ha sa juhuyātprāpnoti haivaitadannādyaṃ ya evaṃ vidvāṃstarhi juhoti

And when the coals are glowing intensely, then, indeed, that (fire) is the Brahman. And if anybody wishes to become endowed with holy lustre (brahmavarkasin), let him offer then: and, assuredly, he who, knowing this, offers then, obtains that food (object).