Negative commandment · Lo Ta’aseh #53 of 613
For a Jewish woman never to marry an Ammonite or Moabite [even after conversion]
שלא יישא עמוני ומואבי בת ישראל לעולם
The verse · Devarim 23:4
לֹא־יָבֹא עַמּוֹנִי וּמוֹאָבִי בִּקְהַל ה׳ גַּם דּוֹר עֲשִׂירִי לֹא־יָבֹא לָהֶם בִּקְהַל ה׳ עַד־עוֹלָם
“An ῾Ammonite or a Mo᾽avite shall not enter into the congregation of the Lord; even to their tenth generation shall they not enter into the congregation of the Lord for ever:”
The Rambam’s words
For a Jewish woman never to marry an Ammonite or Moabite [even after conversion], as [Deuteronomy 23:4] states: "An Ammonite or a Moabite shall never enter the congregation of G-d."
שלא יישא עמוני ומואבי בת ישראל לעולם, שנאמר "לא יבוא עמוני ומואבי, בקהל ה'" (דברים כג,ד).
Who & when
“And this prohibition was practiced before King Sancheriv of Assyria came up against Jerusalem and exiled Israel, and also jumbled all of the nations and mixed them with each other — as he ruled over the whole [known] world.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה