Reference for Shabbat 241:1

טבילה בזמנה לאו מצוה ומהדרינן ור' יוסי סבר טבילה בזמנה מצוה ולא מהדרינן

Tebillah in its [due] time is not obligatory,<span class="x" onmousemove="('comment',' Even an obligatory tebillah need not be performed just when it is due. ');">¹</span> hence we seek [it]; whereas R. Jose holds, Tebillah in its [duel time is obligatory, hence we do not seek [it]. Now, does then R. Jose hold, Tebillah in its [due] time is obligatory? Surely it was taught: A <i>zab</i> and a zabah, a male leper and a female leper, he who cohabits with a <i>niddah</i>,<span class="x" onmousemove="('comment',' Which defiles him — such coition is strictly forbidden. ');">²</span>

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