Reference for Niddah 71:16

תטמא מעת לעת

it suffices for her to reckon her uncleanness from the time of her observation. Now according to Levi who said that there exist two sources one may well concede this ruling since it is for this reason<span class="x" onmousemove="('comment',' That there exist two sources. ');">¹⁴</span>

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