Shatapatha Brahmana 7.1.1.21

7.1.1.21

sa catasraḥ pūrvā upadadhāti | ātmā hyevāgre sambhavataḥ sambhavati dakṣiṇata udaṅṅāsīna uttarārdhyām prathamāmupadadhāti tatho hāsyaiṣo 'bhyātmamevāgniścito bhavati

Four (bricks) he puts on first, for of him that is being produced it is the body (trunk) that is produced first. Sitting south (of the hearth-site) with his face to the north he puts on first one (brick) of the upper (north) part (of the trunk); and in this manner that Agni of his comes to be built up towards (or for the Sacrificer) himself.