Shatapatha Brahmana 7.1.1.11

7.1.1.11

agnerbhasmāsyagneḥ purīṣamasīti | yātayāma vā agnerbhasmāyātayāmnyaḥ sikatā ayātayāmamevainadetatkaroti tābhiḥ sarvaṃ gārhapatyam pracādayati yonirvai gārhapatyā citī retaḥ sikatāḥ sarvasyāṃ tadyonau reto dadhāti

[He scatters it, with Vâg. S. XII, 46] 'Agni's ashes thou art! Agni's soil thou art!' for Agni's ashes are useless, and the sand is not useless: he thus makes it (the Gârhapatya hearth) to be useful. He covers with it the whole Gârhapatya; for the Gârhapatya altar is the womb, and the sand is seed: he thus fills the whole womb with seed.