Shatapatha Brahmana 8.5.3.7

8.5.3.7

atha yā aṣṭāviṣṭakā atiyanti | sāṣṭākṣarā gāyatrī brahma gāyatrī tadyattadbrahmaitattadyadetanmaṇḍalaṃ tapati tadetasminnekaviṃśe pratiṣṭhāyām pratiṣṭhitaṃ tapati tasmānnāvapadyate

And the eight bricks which remain over are the Gâyatrî consisting of eight syllables; but the Gâyatrî is the Brahman, and as to that Brahman, it is yonder burning disk: it burns, while firmly-established on that twenty-one-fold one, as on a foundation, whence it does not fall down.