Reference for Chullin 30:42

נעץ סכין בכותל ושחט בה שחיטתו כשרה היה צור יוצא מן הכותל או קנה עולה מאליו ושחט בו שחיטתו פסולה

the slaughtering is valid; with an implement that was attached to the ground, the slaughtering is valid; if one inserted a knife into a wall and slaughtered, [moving the throat of the animal to and fro across the knife], the slaughtering is valid; if there was a sharp flint jutting from the wall, or a reed growing of itself, and one slaughtered therewith, the slaughtering is invalid.

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