Shatapatha Brahmana 10.1.1.11

10.1.1.11

tadidamannaṃ jagdhaṃ dvedhā bhavati yadasyāmṛtamūrdhvaṃ tannābherūrdhvai prāṇairuccarati tadvāyumapyetyatha yanmartyam parāk tannābhimatyeti taddvayam bhūtvemāmapyeti mūtraṃ ca purīṣaṃ ca tadyadimāmapyeti yo'yamagniścitastaṃ tadapyetyatha yadvāyumapyeti yaścite'gnirnidhīyate taṃ tadapyetyeṣo evātrāpītiḥ

Now, this food, when eaten, becomes of two kinds,--that part of it which is immortal (remains) above the navel: by the upward vital airs it moves upwards and enters the air; but that part of it which is mortal tends to move away: it passes beyond the navel, and, having become twofold, enters this (earth), as urine and faeces. Now that which enters this (earth) enters the fire-altar built here; and that which enters the air enters that fire which is placed on the built (altar). This also is the entering therein.