Shatapatha Brahmana 7.1.1.10

7.1.1.10

yadveva sikatā nivapati | agneretadvaiśvānarasya reto yatsikatā agnimu vā etaṃ vaiśvānaraṃ ceṣyanbhavati na vā aretaskātkiṃ cana vikriyate 'smādretaso 'dhi vikriyātā iti

And again why he scatters sand,--sand is nothing else than the seed of Agni Vaisvânara, and him, Agni Vaisvânara, he is about to build up; but nothing is fashioned from out of the seedless: 'May he (Agni) be fashioned from out of this seed!' so he thinks.