Śatapatha Brāhmaṇa ("of a hundred paths") 6.6.1.6

6.6.1.6

yadvevaitaṃ vaiśvānaraṃ nirvapati | vaiśvānaraṃ vā etamagniṃ janayiṣyanbhavati tametatpurastāddīkṣaṇīyāyāṃ reto bhūtaṃ siñcati yādṛgvai yonau retaḥ sicyate tādṛgjāyate tadyadetamatra vaiśvānaraṃ reto bhūtaṃ siñcati tasmādeṣo 'mutra vaiśvānaro jāyate

And, again, as to why he prepares one for Vaisvânara,--it is because he is about to produce Agni as Vaisvânara (belonging, or dear to, all men): in the initiation offering he first pours him forth as seed, and whatlike the seed is that is poured into the womb, suchlike is (the child) born therefrom; and inasmuch as he now pours forth that (Agni) Vaisvânara as seed, therefore he is born hereafter as Vaisvânara.