Reference for Chullin 3:2

וכולן ששחטו ואחרים רואין אותן שחיטתן כשרה:

AND THEIR SLAUGHTERING IS VALID, EXCEPT A DEAF - MUTE, AN IMBECILE OR A MINOR, LEST THEY INVALIDATE THEIR SLAUGHTERING; AND IF ANY OF THESE SLAUGHTERED WHILE OTHERS WERE STANDING OVER THEM, THEIR SLAUGHTERING IS VALID.

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