Shatapatha Brahmana 9.1.2.37

9.1.2.37

bṛhaduttare pakṣe | dyaurvai bṛhaddyaurhi barhiṣṭhā divamevāsyaitaduttaram pakṣaṃ karotyatho uttaramevāsyaitatpakṣamanasthikamamṛtaṃ karoti

The Brihat (hymn he sings) at the left wing; the Brihat (great), doubtless, is the sky, for the sky is the greatest (of worlds): he thus makes the sky his (Agni's) left wing, and that left wing of his he thus makes boneless and immortal.