Shatapatha Brahmana 10.1.4.9

10.1.4.9

atha vikarṇīṃ ca svayamātṛṇāṃ copadhāya hiraṇyaśakalaiḥ prokṣatyagnimabhyādadhāti rūpameva tatprajāpatirhiraṇmayamantata ātmano'kuruta tadyadantatastasmādidamantyamātmano rūpaṃ tasmādāhurhiraṇmayaḥ prajāpatiriti tathaivaitadyajamāno rūpameva hiraṇmayamantata ātmanaḥ kurute tadyadantatastasmādidamantyamātmano rūpaṃ tasmādye caitadvidurye ca na hiraṇmayo'gnicidamuṣmiṃloke sambhavatītyevāhuḥ

Having then laid down the Vikarnî and Svayamâtrinnâ, he scatters chips of gold, and places the fire thereon: Pragâpati then finally made a golden form for his body; and inasmuch as (he did so) finally, this was the final form of his body; whence people speak of 'the golden Pragâpati.' And in like manner does the Sacrificer now finally make a golden form for his body; and inasmuch as (he does so) finally, this is the final form of his body; and hence, whether they know this or not, people say that the Agnikit (he who has built an altar) is born in yonder world as one made of gold.