Reference for Niddah 120:1

כאן למפרע כאן להבא

for the former applies to retrospective uncleanness<span class="x" onmousemove="('comment',' Sc. to a case where the owner of the shirt discovered the stain on it before the other to whom she had lent it had discovered the stain on her own under garment, Though the other subsequently discovered the stain, she cannot be regarded as unclean retrospectively (from the time the owner of the shirt had discovered the stain) since at that time she was still in a condition of cleanness (cf. Tosaf. and Tosaf. Asheri, contra Rashi). ');">¹</span>

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