Śatapatha Brāhmaṇa ("of a hundred paths") 6.1.2.36

6.1.2.36

tadāhuḥ | kasmai kāmāyāgniścīyata iti suparṇo mā bhūtvā divaṃ vahādityu haika āhurna tathā vidyādetadvai rūpaṃ kṛtvā prāṇāḥ prajāpatirabhavannetadrūpaṃ kṛtvā prajāpatirdevānasṛjataitadrūpaṃ kṛtvā devā amṛtā abhavaṃstadyadevaitena prāṇā abhavanyatprajāpatiryaddevāstadevaitena bhavati

As to this they say, 'For what object is this fire (altar) built? Having become a bird, he (Agni) shall bear me to the sky!' so say some; but let him not think so; for by assuming that form, the vital airs became Pragâpati; by assuming that form, Pragâpati created the gods; by assuming that form, the gods became immortal and what thereby the vital airs, and Pragâpati, and the gods became, that indeed he (the Sacrificer) thereby becomes.