Reference for Chullin 34:23

מנין לבדיקת סכין מן התורה

the knife is usually examined by the light of the sun.<span class="x" onmousemove="('comment',' In order to detect any notches; either by holding up the knife to the light of the sun or by watching the shadow of the knife on the ground.');">¹⁴</span>

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