Śatapatha Brāhmaṇa ("of a hundred paths") 1.8.2.11

1.8.2.11

sa vai khalu barhiḥ prathamaṃ yajati | ayaṃ vai loko barhiroṣadhayo barhirasminnevaitalloka oṣadhīrdadhāti tā imā asmiṃloka oṣadhayaḥ pratiṣṭhitāstadidaṃ sarvaṃ jagadasyāṃ teneyaṃ jagatī tajjagatīm prathamāmakurvan

In the first place, then, he offers to the Barhis. The Barhis indeed is this world; the Barhis is the plants: hence he thereby bestows plants on this world, and these plants are firmly rooted in this world. Now this entire universe (gagat) is contained in this (metre), and therefore the latter is (called) gagatî: this is why they have placed the gagatî metre first.