Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.1.36

7.1.1.36

atha purīṣaṃ nivapati | tasyopari bandhustaccātvālavelāyā āharatyagnireṣa yaccātvālastatho hāsyaitadāgneyameva bhavati sā samambilā syāttasyokto bandhuḥ

He then throws thereon a layer of earth,--the significance of this (will be told) further on. He takes it from the edge of the pit (Kâtvâla); for the Kâtvâla is the same as Agni, and in this way does that which is of Agni's nature become his. It (the Gârhapatya altar) should be even with the mouth (of the fire-pan): the significance of this has been told.