Shatapatha Brahmana 6.2.1.29

6.2.1.29

yadveva dvādaśa | dvādaśākṣarā vai jagatīyaṃ vai jagatyasyāṃ hīdaṃ sarvaṃ jagadiyamu vā agnirasyai hi sarvo 'gniścīyate yāvānagniryāvatyasya mātrā tāvataivainametadāprīṇāti

And, again, why there are twelve,--of twelve syllables consists the Gagatî, and the Gagatî is this earth, for on her there is everything that moves (gag-at) here. And Agni also is this earth, for it is out of her that the whole fire (altar) is built up: as great as Agni is, as great as is his measure, by so much he thus propitiates him.