Shatapatha Brahmana 10.2.1.8

10.2.1.8

atha rūpamūttamaṃ karoti atraiṣa sarvo'gniḥ saṃskṛtastasmindevā etadrūpamuttamamadadhustathaivāsminnayametadrūpamuttamaṃ dadhāti sa sahasramṛjvālikhitā iṣṭakāḥ karoti sahasramityālikhitāḥ sahasramityālikhitāḥ

He now gives to it (the altar) the highest form. This Agni had now been completely restored, and the gods conferred upon him this highest form; and in like manner does this (Sacrificer) confer upon him this highest form: he makes a thousand bricks marked with straight lines, a thousand marked this way (from left to right), and a thousand marked that way (from right to left).