Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.2.23

7.1.2.23

tadāhuḥ | yadayaṃ loko gārhapatyo 'ntarikṣaṃ dhiṣṇyā dyaurāhavanīyo 'ntarikṣaloka u asmāllokādanantarhito 'tha kasmādgārhapatyaṃ citvāhavanīyaṃ cinotyatha dhiṣṇyāniti saha haivemāvagre lokāvāsatustayorviyatoryo 'ntareṇākāśa āsīttadantarikṣamabhavadīkṣaṃ haitannāma tataḥ purāntarā vā idamīkṣamabhūditi tasmādantarikṣaṃ tadyadgārhapatyaṃ citvāhavanīyaṃ cinotyetau hyagre lokāvasṛjyetāmatha pratyetya dhiṣṇyānnivapati karmaṇa evānantarayāyātho antayorvāva saṃskriyamāṇayormadhyaṃ saṃskriyate

As to this they say, 'As the Gârhapatya is this (terrestrial) world, the Dhishnya hearths the air, and the Âhavanîya the sky, and the air-world is not separated from this (earth-) world, why then, after building the Gârhapatya, does he build the Âhavanîya, and (only) then the Dhishnyas?' Well, at first these two worlds (heaven and earth) were together; and when they parted asunder, the space which was between (antar) them became that air (antariksha); for 'îksha ' indeed it was theretofore, and 'Now this "îksha" has come between (antarâ),' they said, whence 'antariksha' (air). And as to why, after building the Gârhapatya, he builds the Âhavanîya, it is because these two worlds were created first. Then, going back, he throws up the Dhishnya hearths, just to prevent discontinuity of the sacred work; and thus indeed the middle is completed, after the two ends have been completed.