Reference for Sanhedrin 152:16

ההוא גברא דמצמצמא לחיותה דחבריה בשימשא ומתה רבינא מחייב רב אחא בר רב פטר

— Because iron can kill no matter what its size. It has been taught likewise: Rabbi said; It was well known to Him who spake and the world came into being that iron, no matter how small, can kill; therefore the Torah prescribed no size for it. This however, is only if one pierced therewith:<span class="x" onmousemove="('comment',' But if used to strike therewith, it must be of a certain minimum size before the murderer is executed. ');">¹⁴</span>

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