Shatapatha Brahmana 4.1.1.20

4.1.1.20

athopaniṣkrāmati | urvantarikṣamanvemītyantarikṣaṃ vā anu rakṣaścaratyamūlamubhayataḥ paricinnaṃ yathāyam puruṣo 'mūla ubhayataḥ paricinno 'ntarikṣamanucaratyetadvai yajurbrahma rakṣohā sa etena brahmaṇāntarikṣamabhayamanāṣṭraṃ kurute

He then walks out (of the Havirdhâna) with, 'I walk along the wide air.' For along the air the Rakshas roams rootless and unfettered on both sides, even as man here roams along the air, rootless and unfettered on both sides; and, that formula being the Brahman (prayer), a slayer of the Rakshas, he, by means of that Brahman, renders the air free from danger and injury.