Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.1.5

9.2.1.5

yadveva svayamātṛṇāṃ vyāghārayati | uttaravedirhaiṣāgneratha yāmamūm pūrvāṃ vyāghārayatyadhvarasya sātha haiṣāgnestāmetadvyāghārayati

And, again, as to why he offers on the naturally-perforated one;--this (brick) is an uttara-vedi (high-altar) of Agni (the fire-altar); and that former libation which he makes belongs to the Soma-sacrifice, but this one belongs to the fire-altar; this he now offers.