Reference for Shevuot 35:5

כניסה תנינא נמצא על שלו טמאין וחייבין בקרבן מאי לאו בסמוך לוסתה ואכניסה

Entering, we have learnt' [in another Mishnah] - If [blood is] found on his [rag after cohabitation], they are [both] unclean,<span class="x" onmousemove="('comment',' Seven days; Lev. XV, 19 and 24.');">⁸</span> and are liable for a sacrifice.<span class="x" onmousemove="('comment',' Sin offering for cohabiting while she is unclean. Nid. 14a.');">⁹</span> Now this surely refers [does it not? ] to the time near her period, and to [the act of] entering.<span class="x" onmousemove="('comment',' Hence we learn that for entering (near her period) he is liable for a sacrifice, if she becomes unclean.');">¹⁰</span>

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