Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.38

6.2.2.38

tadāhuḥ | naitasya paśoḥ samiṣṭayajūṃṣi juhuyānna hṛdayaśūlenāvabhṛthamabhyaveyādārambho vā eṣo 'gneḥ paśurvyavasargo devatānāṃ samiṣṭayajūṃṣi saṃsthāvabhṛtho nedārambhe devatā vyavasṛjāni nedyajñaṃ saṃsthāpayānīti sa vai sameva sthāpayedetena paśuneṣṭvā tatprajāpatirapaśyadyathaitasyāgnerantaṃ na paryaittasmātsaṃsthāpayedyadveva saṃsthāpayati prāṇa eṣa paśustasya yadantariyātprāṇasya tadantariyādyadu vai prāṇasyāntariyāttata evam mriyeta tasmātsameva sthāpayedathāto vratānāmeva

As to this they say, 'Of that animal he should offer no Samishtayagus, nor should he go down with the heart-spit to the purificatory bath; for that animal (sacrifice) is the commencement of Agni; the Samishtayagus are the gracious dismissal of the deities; and the purificatory bath is the completion;--lest he should at the very commencement dismiss the deities, and complete the sacrifice.' Let him nevertheless complete (the sacrifice): Pragâpati, having offered that animal, saw that he had not reached the end of him, Agni,--let him therefore complete (the sacrifice). And, again, why he completes it;--that animal sacrifice is his vital air, and if anything were to cut him off from that, it would cut him off from the vital air; and if anything were to cut him off from the vital air, he would thus die: let him therefore complete (the sacrifice). Now, then, as to the vows (rites of abstinence).