Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.2.20

7.1.2.20

atha ye ete dve yajuṣī | etattadyadantarāhavanīyaṃ ca gārhapatyaṃ ca tadantarikṣaṃ sa ātmā tadyatte dve bhavatastasmādetattanīyo yadantarāhavanīyaṃ ca gārhapatyaṃ ca tasmādeṣāṃ lokānāmantarikṣalokastaniṣṭhaḥ

And as to those two formulas, they are that space between the Âhavanîya and the Gârhapatya, that air (-world), that body (of Pragâpati). And because there are two of them (making up one anushtubh), therefore that space (and hearth) between the Âhavanîya and the Gârhapatya (viz. the Âgnîdhrîya hearth) is smaller; and therefore the air-world is the smallest of these worlds.