Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.6.18

10.2.6.18

athādhyātmam yadeva puruṣe'nnaṃ tadannaṃ yā āpastatpānamasthīnyeva śrīstaddhi pariśritāṃ rūpam majjāno jyotistaddhi yajuṣmatīnāṃ rūpam prāṇo'mṛtaṃ taddhyagne rūpam prāṇo'gniḥ prāṇo'mṛtamityu vā āhuḥ

Now as to the body. Whatever food there is in man, that is his food; whatever water, that is his water; his well-being (safety, strength) is the bones, for they are of the nature of enclosing-stones; his light the marrow, for that is of the nature of the yagushmatî (bricks); his immortal element the breath, for that is of the nature of fire;--and, indeed, people say, 'The breath is fire, the breath is the immortal.'