Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.42

7.3.1.42

yadveva sikatā nivapati | prajāpatireṣo 'gniḥ sarvamu brahma prajāpatistaddhaitadbrahmaṇa utsanne yatsikatā atha yadanutsannamidaṃ tadyo 'yamagniścīyate tadyatsikatā nivapati yadeva tadbrahmaṇa utsannaṃ tadasminnetatpratidadhāti tā asaṃkhyātā aparimitā nivapati ko hi tadveda yāvattadbrahmaṇa utsannaṃ sa ha vā etaṃ sarvaṃ kṛtsnam prajāpatiṃ saṃskaroti ya evaṃ vidvāntsikatā nivapati

And as to why he scatters sand,--that Agni (fire-altar) is Pragâpati, and Pragâpati is the whole Brahman. Now that sand is (put) in (the place of) the lost part of the Brahman; and that part of it which has not been lost is this fire-altar which is now being built: thus when he scatters sand he restores to him that lost part of the Brahman. That (sand which) he scatters is unnumbered, unlimited; for who knows how great is that lost part of the Brahman? And verily he who, knowing this, scatters sand, restores the whole, complete Pragâpati.