Reference for Shevuot 34:13

הוי בה רבה במאי אילימא סמוך לוסתה ובמאן אילימא בתלמיד חכם בשלמא אכניסה ליחייב קסבר יכולני לבעול אלא אפרישה אמאי ליחייב מזיד הוא

And with whom? Shall we say, a learned man? Granted, then, for entering he should be liable, for he thought I am able to cohabit;<span class="x" onmousemove="('comment',' Before she has her period; if, therefore, she becomes unclean during cohabitation, he commits a sin unwittingly, and must bring a sin offering.');">²⁴</span> but for withdrawing, why should he be liable, since he acted wilfully!<span class="x" onmousemove="('comment',' Being learned, he knows that it is prohibited to withdraw immediately, and is therefore liable for kareth, and not a sin offering.');">²⁵</span>

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