Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.33

6.2.2.33

yadvevāṣṭācatvāriṃśat | aṣṭācatvāriṃśadakṣarā vai jagatī jagatī sarvāṇi candāṃsi sarvāṇi candāṃsi prajāpatiḥ prajāpatiragniryāvānagniryāvatyasya mātrā tāvattadbhavati

And, again, why there are forty-eight;--of forty-eight syllables consists the Gagatî; the Gagatî (comprises) all the metres; all the metres are Pragâpati (the sacrifice); and Pragâpati is Agni as great as Agni is, as great as is his measure, so great does this become.