Shatapatha Brahmana 3.8.4.5

3.8.4.5

sa ha tveva paśumālabheta | ya enam medhamupanayedyadi kṛśaḥ syādyadudaryasya medasaḥ pariśiṣyata tadgude nyṛṣetprāṇo vai gudaḥ so 'yam prāṅātatastamayam prāṇo 'nusaṃcarati prāṇo vai paśuryāvaddhyeva prāṇena prāṇiti tāvatpaśuratha yadāsmātprāṇo 'pakrāmati dārveva tarhi bhūto 'narthyaḥ śete

He alone, however, may slay an animal who can supply it with the sacrificial essence. And if it be lean, let him stuff into the hind-part whatever may be left of the fat of the belly: the hind-part being (an opening of) the vital air, and that (animal) extending from thence forward, that vital air pervades it all through. The animal, forsooth, is breath; for only so long (does) the animal (live), as it breathes with the breath; but when the breath departs from it, it lies there useless, even (as) a block of wood.