Shatapatha Brahmana 3.8.1.15

3.8.1.15

atha punaretyāhavanīyamabhyāvṛtyāsate | nedasya saṃjñapyamānasyādhyakṣā asāmeti tasya na kūṭena praghnanti mānuṣaṃ hi tanno eva paścātkarṇam pitṛdevatyaṃ hi tadapigṛhya vaiva mukhaṃ tamayanti veṣkaṃ vā kurvanti tannāha jahi mārayeti mānuṣaṃ hi tatsaṃjñapayānvaganniti taddhi devatrā sa yadāhānvagannityetarhi hyeṣa devānanugacati tasmādāhānvaganniti

They then step back (to the altar) and sit down turning towards the Âhavanîya, 'lest they should be eye-witnesses to its being quieted (strangled).' They do not slay it on the frontal bone, for that is human manner; nor behind the ear, for that is after the manner of the Fathers. They either choke it by merely keeping its mouth closed, or they make a noose. Therefore he says not, 'Slay! kill!' for that is human manner, but, 'Quiet it! It has passed away!' for that is after the manner of the gods. For when he says, 'It has passed away,' then this one (the Sacrificer) passes away to the gods: therefore he says, 'It has passed away.'