Reference for Niddah 118:57

ג' נשים שלבשו חלוק אחד או שישבו על ספסל אחד ונמצא עליו דם כולן טמאות

or where she was sitting, since only one doubt is involved, but not where a double doubt is involved;<span class="x" onmousemove="('comment',' Whether (a) the blood issued from the woman or the man and (b) if from the woman whether from the menstrual source or from some internal wound. ');">⁵¹</span>

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