Reference for Shevuot 35:8

א"ל רבא ציית מאי דקאמר רבך היכי מצית דתניא עלה זו היא מצות עשה שבנדה שחייבין עליה ואם איתא מצות לא תעשה היא

he teaches us also [the law concerning] 'If found on his.'<span class="x" onmousemove="('comment',' Though this is superfluous.');">¹⁷</span> Said Rabina to R'Adda; How can you maintain that that [other Mishnah] refers to the time not near her period, and to withdrawing, seeing that it states; If [blood is] found, and found implies later;<span class="x" onmousemove="('comment',' After withdrawing, blood was found, but during cohabitation they were not aware of uncleanness.');">¹⁸</span> and if it refers to withdrawing, from the very first when he withdrew he already had the knowledge!<span class="x" onmousemove="('comment',' That she is unclean, for she told him during cohabitation.');">¹⁹</span>

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