Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.2.25

7.4.2.25

atha viśvajyotiṣamupadadhāti | agnirvai prathamā viśvajyotiragnirhyevāsmiṃloke viśvaṃ jyotiragnimevaitadupadadhāti tāmanantarhitāṃ retaḥsigbhyāmupadadhātīmau vai lokau retaḥsicāvanantarhitaṃ tadābhyāṃ lokābhyāmagniṃ dadhātyantarevopadadhātyantareva hīmau lokāvagniḥ

He then lays down a Visvagyotis (all-light brick);--the first 'all-light' (brick) is Agni, for Agni is all the light in this (terrestrial) world: it is Agni he thus lays down. He lays it down close to the seed-shedding ones,--the seed-shedding ones being these two worlds, he thus places Agni together with these two worlds. He lays it down between (the two Retahsik), for Agni (the fire) is within these two worlds.