Reference for Bava Kamma 180:17

אלא מה אגרוף מיוחד שהוא מסור לעדים אף כל שהוא מסור לעדים

Simeon the Temanite remarked that just as a fist is a concrete object that can be submitted for examination to the assembly of the judges and the witnesses, so also it is necessary that all other instruments should be able to be submitted [for consideration] to the assembly of the judges and the witnesses, which excludes the case where the instrument of killing disappeared from under the hands of the witnesses.<span class="x" onmousemove="('comment',' [Though the witnesses had an opportunity of examining the killing instrument, where the Judges had no such opportunity, no death penalty can be passed.] ');">¹³</span>

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