Reference for Chullin 58:7

ומאחר דאפילו פסולא דרבנן ליכא למה לי למרק

Raba remarked: All agree that where a gentile cut the first organ of the throat and an Israelite the second, the slaughtering is invalid, for the animal has already been rendered trefah by the hand of the gentile.<span class="x" onmousemove="('comment',' For if the cutting of the first organ is considered an act of shechitah the slaughtering here is invalid because it has been done by a gentile (v. supra 13a) ; and if it is not considered an act of shechitah then it can only be regarded as a mutilation of the organ, a defect which renders the animal trefah and any subsequent slaughtering invalid.');">³</span>

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