Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.32

6.2.2.32

tato yāṣṭācatvāriṃśat | sāṣṭācatvāriṃśadakṣarā jagatīyaṃ vai jagatyasyāṃ hīdaṃ sarvaṃ jagadiyamu vā agnirasyai hi sarvo 'gniścīyate yāvānagniryāvatyasya mātrā tāvattadbhavati

Now what forty-eight there are (in these fifty-eight), they are the Gagatî (metre) consisting of forty-eight syllables;--the Gagatî doubtless is this earth, for it is thereon that everything is that moves (gagat); and Agni also is this earth, for it is thereof that the whole Agni is built up: as great as Agni is, as great as is his measure, so great does this become.