Reference for Chullin 38:10

במקום שחיטה מאי למימרא

R'Abba was once sitting behind R'Kahana whilst R'Kahana was before Rab Judah, when R'Kahana asked: What is the law if a slaughterer first cut a third [of the windpipe] within the prescribed area, another third outside it and the last third within it? - Rab Judah answered: The slaughtering is valid.

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