Reference for Chullin 60:39

תנא דבי רבי ישמעאל

THESE PROVISIONS APPLY ONLY TO THE CASE WHERE THE SLAUGHTERER MOVED THE KNIFE FORWARD AND NOT BACKWARD, OR BACKWARD AND NOT FORWARD; BUT IF HE MOVED THE KNIFE TO AND FRO, HOWEVER SMALL IT WAS, EVEN IF IT WAS A LANCET, THE SLAUGHTERING IS VALID.

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