Śatapatha Brāhmaṇa ("of a hundred paths") 13.8.3.6

13.8.3.6

atha trayodaśa pādamātrya iṣṭakā alakṣaṇāḥ kṛtā bhavanti yā evāmūragnāviṣṭakāstā etā yajuṣā tā upadadhāti tūṣṇīmimā daivaṃ caiva tatpitryaṃ ca vyākaroti

Now thirteen unmarked bricks, measuring a foot (square), have been made: they are just like those bricks in the fire-altar. Those (altar bricks) he lays down with a formula, silently these: he thus keeps the divine separate from what belongs to the Fathers.