Reference for Shevuot 2:4

וְאֵיזוֹ הִיא מִצְוַת עֲשֵׂה שֶׁבַּנִּדָּה שֶׁחַיָּבִין עָלֶיהָ, הָיָה מְשַׁמֵּשׁ עִם הַטְּהוֹרָה וְאָמְרָה לוֹ נִטְמֵאתִי, וּפֵרַשׁ מִיָּד, חַיָּב, מִפְּנֵי שֶׁיְּצִיאָתוֹ הֲנָאָה לוֹ כְּבִיאָתוֹ:

And which is a positive commandment of niddah [comparable to that of the sanctuary, where the uncleanliness occurs after permitted entrance], where they [beth-din] are liable [for a bullock of forgetfulness if they erred in the ruling]? If he were cohabiting with a clean (non-niddah) woman [having "entered" permittedly], and she said to him: "I have become unclean" [now], and he withdrew immediately [with erect organ], he is liable, for his withdrawal is pleasurable to him as is his entry. [But he should wait without movement until the organ "dies" and withdraw when he has lost his erection. This is the positive commandment of niddah.]

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