Shatapatha Brahmana 6.1.2.29

6.1.2.29

tadāhuḥ | kasmādasyā agniścīyata iti yatra vai sā devatā vyasraṃsata tadimāmeva rasenānu vyakṣarattaṃ yatra devāḥ samaskurvaṃstadenamasyā evādhi samabharantsaiṣaikaiveṣṭakeyameveyaṃ hyagnirasyai hi sarvo 'gniścīyate seyaṃ catuḥsraktirdiśo hyasyai sraktayastasmāccatuḥkraktaya iṣṭakā bhavantīmāṃ hyanu sarvā iṣṭakāḥ

Here now they say, 'Wherefore is Agni (the fire-altar) built of this (earth)?' But, surely, when that deity (Pragâpati) became relaxed (fell asunder), he flowed along this (earth) in the shape of his life-sap; and when the gods restored him (put him together), they gathered him up from this earth: this earth then is that one brick, for Agni is this earth, since it is thereof that the whole Agni is built up. Now this earth is four-cornered, for the quarters are her corners: hence the bricks are four-cornered; for all the bricks are after the manner of this earth.