Shatapatha Brahmana 8.5.3.6

8.5.3.6

tā vā etāḥ | ekaviṃśatiriṣṭakā ime ca lokā diśaśceme ca vai lokā diśaśca pratiṣṭheme ca lokā diśaścaikaviṃśastasmādāhuḥ pratiṣṭhaikaviṃśa iti

These twenty-one bricks, then, are these worlds and the regions, and these worlds and the regions are a foundation, and these worlds and the regions are twenty-one: whence they say, 'the Ekavimsa (twenty-one-fold) is a foundation.'