Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.9

7.3.1.9

athāhavanīya eva puṣkaraparṇamupadadhāti | na gārhapatya āpo vai puṣkaraparṇaṃ dyaurāhavanīyo divi tadapo dadhātyubhayatra sikatā nivapati reto vai sikatā ubhayatra vai vikriyate tasmādretaso 'dhi vikriyātā iti

And only on the Âhavanîya (site) he places a lotus leaf, not on the Gârhapatya; for the lotus leaf means water, and the Âhavanîya the sky: he thus places the waters (vapours) in the sky. On both he scatters sand; for sand means seed, and in both (fire-altars) fashioning (of Agni) takes place: 'May he be fashioned from out of that seed!' thus he thinks.