Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.2.22

7.4.2.22

atha retaḥsicā upadadhāti | imau vai lokau retaḥsicāvimau hyeva lokau retaḥ siñcata ito vā ayamūrdhvaṃ retaḥ siñcati dhūmaṃ sāmutra vṛṣṭirbhavati tāmasāvamuto vṛṣṭiṃ tadimā antareṇa prajāyante tasmādimau lokau retaḥsicau

He then lays down two Retahsik (seed-shedding bricks);--the seed-shedders doubtless are these two worlds, for these two worlds do shed seed;--this (terrestrial world) sheds seed upwards from here (in the form of) smoke; it becomes rain in yonder world, and that rain yonder world (sheds) from above: hence (creatures) are born within these two worlds, and therefore these two worlds are seed-shedders.