Reference for Chullin 105:27

אין דרוסה לשועל

Because although most lions attack with their claws there are a few that do not; moreover, all that do claw do not usually lose a nail, therefore the fact that this ox has a nail lodged in its back suggests that had rubbed itself against a wall.<span class="x" onmousemove="('comment',' It is so rare an occurrence for a lion to lose a nail while attacking with its claw, that it is much more probable to suggest that the animal got the nail lodged in its back from having scratched itself against a wall in which this nail protruded.');">⁸</span>

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