Mishnayos Yevamos Perek 8 Mishnah 3

Ammonite and Moabite converts are prohibited from entering into the congregation and marrying a woman who was born Jewish, and their prohibition is eternal, for all generations. However, their female counterparts, even the convert herself, are permitted immediately. Egyptian and Edomite converts are prohibited from entering into the congregation only for three generations, both males and females. Rabbi Shimon renders permitted Egyptian and Edomite females immediately. Rabbi Shimon said: The matter may be derived by way of an a fortiori inference: If in a place where the Torah rendered prohibited the males with an eternal prohibition, i.e., Ammonites and Moabites, it rendered permitted the females immediately, then in a place where it rendered prohibited the males for only three generations, i.e., Egyptians and Edomites, is it not right that we should render permitted the females immediately? Rabbi Shimon’s colleagues said to him: If you are reporting a halakha that you received from your teachers, we will accept it from you. But if you merely wish to prove your case with an a fortiori inference based on your own reasoning, there is a refutation of your argument. Rabbi Shimon said to them: That is not so. I disagree with your claim that the a fortiori inference can be refuted, but in any case I am stating a halakha handed down to me by my teachers. Mamzerim and the Gibeonites who converted to Judaism in the days of Joshua are prohibited from entering into the congregation and marrying a woman who was born Jewish. Their prohibition is eternal, for all generations, and it applies to both males and females.

עַמּוֹנִי וּמוֹאָבִי, אֲסוּרִים, וְאִסּוּרָן אִסּוּר עוֹלָם, אֲבָל נְקֵבוֹתֵיהֶם מֻתָּרוֹת מִיָּד. מִצְרִי וַאֲדוֹמִי אֵינָם אֲסוּרִים אֶלָּא עַד שְׁלֹשָׁה דוֹרוֹת, אֶחָד זְכָרִים וְאֶחָד נְקֵבוֹת. רַבִּי שִׁמְעוֹן מַתִּיר אֶת הַנְּקֵבוֹת מִיָּד. אָמַר רַבִּי שִׁמְעוֹן, קַל וָחֹמֶר הַדְּבָרִים, וּמָה אִם בִּמְקוֹם שֶׁאָסַר אֶת הַזְּכָרִים אִסּוּר עוֹלָם, הִתִּיר אֶת הַנְּקֵבוֹת מִיָּד, מְקוֹם שֶׁלֹּא אָסַר אֶת הַזְּכָרִים אֶלָּא עַד שְׁלֹשָׁה דוֹרוֹת, אֵינוֹ דִין שֶׁנַּתִּיר אֶת הַנְּקֵבוֹת מִיָּד. אָמְרוּ לוֹ, אִם הֲלָכָה נְקַבֵּל, וְאִם לַדִּין, יֵשׁ תְּשׁוּבָה. אָמַר לָהֶם, לֹא כִי, הֲלָכָה אֲנִי אוֹמֵר. מַמְזֵרִין וּנְתִינִין, אֲסוּרִין, וְאִסּוּרָן אִסּוּר עוֹלָם, אֶחָד זְכָרִים, וְאֶחָד נְקֵבוֹת: