Shatapatha Brahmana 6.7.1.11

6.7.1.11

yadvevoparinābhi | yadvai prāṇasyāmṛtamūrdhvaṃ tannābherūrdhvaiḥ prāṇairuccaratyatha yanmartyam parāktannābhimatyeti tadyadeva prāṇasyāmṛtaṃ tadenametadabhisampādayati tenainametadbibharti

And, again, why over the navel,--that part of the vital air which is immortal is above the navel, and streams out by upward breathings; but that which is mortal passes by and away from the navel: he thus makes him (the Sacrificer) obtain the part of the vital air which is immortal, and by that he then carries it (the fire).