Śatapatha Brāhmaṇa ("of a hundred paths") 8.2.1.1

8.2.1.1

dvitīyāṃ citimupadadhāti | etadvai devāḥ prathamāṃ citiṃ citvā samārohannayaṃ vai lokaḥ prathamā citirimameva tallokaṃ saṃskṛtya samārohan

He lays down the second layer. For now the gods, having laid down the first layer, mounted it. But, indeed, the first layer is this (terrestrial) world: it is this same world which, when completed, they mounted.