Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.1.30

6.2.1.30

yadveva dvādaśa | dvādaśākṣarā vai jagatī jagatī sarvāṇi candāṃsi sarvāṇi candāṃsi prajāpatiḥ prajāpatiragniryāvānagniryāvatyasya mātrā tāvataivainametadāprīṇāti

And, again, why there are twelve,--of twelve syllables consists the Gagatî, and the Gagatî is all the metres, and all the metres are Pragâpati (the sacrifice), and Pragâpati is Agni: as great as Agni is, as great as is his measure, by so much he thus propitiates him.