Śatapatha Brāhmaṇa ("of a hundred paths") 8.7.2.18

8.7.2.18

athāta iṣṭakānāmevāvapanasya | yāṃ kāṃ ca yajuṣmatīmiṣṭakāṃ vidyāttām madhyamāyāṃ citā upadadhyādantarikṣaṃ vai madhyamā citirantarikṣamu vai sarveṣām bhūtānāmāvapanamatho annaṃ vai yajuṣmatya iṣṭakā udaram madhyamā citirudare tadannaṃ dadhāti

Now, then, of the location of (special) bricks. Any (special) brick he knows, provided with a formula, let him place in the middle (third) layer; for the middle layer is the air, and the air, doubtless, is the location of all beings. Moreover, bricks with (special) formulas are food, and the middle layer is the belly: he thus puts food into the belly.