Jerusalem Talmud Shevuot 2.4

משנה: אֵי זוֹ הִיא מִצְוַת עֲשֵׂה שֶׁבַּנִּידָּה שֶׁחַיָּיבִין עָלֶיהָ. הָיָה מְשַׁמֵּשׁ עִם הַטְּהוֹרָה וְאָמְרָה לוֹ נִיטְמֵאתִי וּפֵרַשׁ מִיָּד חַיָּיב שֶׁיְּצִיאָתוֹ הֲנָיָיה לוֹ כְּבִיאָתוֹ:

MISHNAH: What is the positive commandment about the menstruating woman? If he was having sex with a pure one and she said to him, I became impure, if he separates immediately he is liable, for his separation is as pleasurable to him as his entry.

הלכה: אֵי זוֹ הִיא מִצְוַת עֲשֵׂה שֶׁבַּנִּידָּה כול׳. כָּהֲנָא אָמַר. אִי אֵיפְשַׁר שֶׁלֹּא הָיְתָה בָהּ יְדִיעָה בַּתְּחִילָּה וִידִיעָה בַסּוֹף וְהֶעֱלֵם בֵּנְתַיִים. הָתִיב רִבִּי שְׁמוּאֵל בַּר אֶבוּדַּיְמִי קוֹמֵי רִבִּי מָנָא. וְיִהְיוּ כֵן הַמּוֹרִים. אָמַר לֵיהּ. אֲנָן בָּעֵיי נִכְנָסִין וְאַתְּ מַיְיתֵי לָן מוֹרִין. מַאי כְדוֹן. אָמַר רִבִּי שְׁמוּאֵל בַּר רַב יִצְחָק. מִצְוֹת מִצְוֹת. מַה מִצְוֹת שֶׁנֶּאֱמַר לְהַלָּן בִּקְבוּעַ אַף כָּאן בִּקְבוּעַ.

HALAKHAH: “What is the positive commandment about the menstruating woman,” etc. Cahana said, is it impossible that there was knowledge at the beginning and knowledge at the end and oblivion in between? Rebbi Samuel ben Eudaimon objected before Rebbi Mana: Could that be for the teachers? He told him, we have a problem with those entering and you bring us teachers? What about it? Rebbi Samuel bar Rav Isaac said, commandments, commandments. Since commandments mentioned there are about fixed-value [offerings], here also about fixed-value [offerings].