Śatapatha Brāhmaṇa ("of a hundred paths") 8.7.2.11

8.7.2.11

sa vai svayamātṛṇāṃ lokampṛṇayā pracādayati | prāṇo vai svayamātṛṇādityo lokampṛṇā prāṇaṃ tadādityena saminddhe tasmādayamuṣṇaḥ prāṇastayā sarvamātmānam pracādayati sarvaṃ tadātmānamādityena saminddhe tasmādayaṃ sarva evātmoṣṇastaddhaitadeva jīviṣyataśca mariṣyataśca vijñānamuṣṇa eva jīviṣyañcīto mariṣyan

The Svayamâtrinnâ (naturally-perforated brick) he encloses with Lokamprinâ (bricks); for the naturally-perforated one is the breath, and the 'space-filler' is the sun: he thus kindles the breath by means of the sun, whence this breath (of ours) is warm. With that (kind of brick) he fills up the whole body: he thereby kindles the whole body by means of the sun, whence this whole body (of ours) is warm. And this, indeed, is the difference between one that will live and one that will die: he that will live is warm, and he that will die is cold.