Reference for Niddah 120:16

אמר אביי

— It might have been presumed that a restriction shall be imposed on its back as a preventive measure against the possible relaxation of the law in regard to its inside,<span class="x" onmousemove="('comment',' Which is susceptible to uncleanness, and a stain on which would in accordance with Rabbinic law subject a woman to uncleanness. ');">¹⁷</span>

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