Shatapatha Brahmana 8.5.4.12

8.5.4.12

tā u vai tisra eva | ayameva lokaḥ prathamā citirdyauruttamātha yā etāstisrastadantarikṣaṃ tadvā idamekamivaivāntarikṣaṃ tā evaṃ tisra evam pañcaivaṃ ṣaḍevaṃ sapta

And, indeed, there are only three of them,--the first layer is this very (terrestrial) world; and the uppermost (layer) is the sky; and those three (intermediate layers) are the air, for there is, as it were, only one air here: thus (there are) three, or five, or six, or seven of them.