Shatapatha Brahmana 8.6.3.15

8.6.3.15

yadveva gārhapatyaṃ ca punaścitiṃ copadadhāti | vediśca haite agneruttaravediścātha ye amū pūrve nivapatyadhvarasya te atha haite agnestadyadete upadhāyāgniṃ nidadhāti vedau caivainametaduttaravedau ca pratiṣṭhāpayati

And as to why he lays down both the Gârhapatya and the Punaskiti. These two, doubtless, are the Vedi and the Uttaravedi (high-altar) of Agni. Now those two former (altars of this kind) which he throws up belong to the Soma-sacrifice, but these belong to the fire-altar; and when, after laying down these two; he deposits Agni (the fire) thereon, then he establishes him both on the Vedi and the Uttaravedi.