Shatapatha Brahmana 8.5.3.5

8.5.3.5

tadyāstisraḥ prathamāḥ | ayaṃ sa loko yā dvitīyā antarikṣaṃ tadyāstṛtīyā dyauḥ sā yāścaturthyaḥ prācī sā digyāḥ pañcamyo dakṣiṇā sā yāḥ ṣaṣṭhyaḥ pratīcī sā yāḥ saptamya udīcī sā

The first three (bricks) are this (terrestrial) world, the second (three) the air, and the third (three) the sky, the fourth the eastern, the fifth the southern, the sixth the western, and the seventh the northern regions.