Shatapatha Brahmana 10.1.2.1

10.1.2.1

prajāpatirimāṃlokānaipsat sa etaṃ vayovidhamātmānamapaśyadagniṃ taṃ vyadhatta tenemaṃ lokamāpnotsa dvitīyaṃ vayovidhamātmānamapaśyanmahāvrataṃ tadvyadhatta tenāntarikṣamāpnotsa tṛtīyaṃ vayovidhamātmānamapaśyanmahadukthaṃ tadvyadhatta tena divamāpnot

Pragâpati was desirous of gaining these worlds. He saw this bird-like body, the Fire-altar: he fashioned it, and thereby gained this (terrestrial) world. He saw a second bird-like body, the (chant of the) Great Rite: he fashioned it, and thereby gained the air. He saw a third bird-like body, the Great Litany: he fashioned it, and thereby gained the sky.