Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 54
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 54, with English translation by Maimonides (Rambam).
שלא להרחיק זרע עשיו מן הקהל אלא עד שלושה דורות, שנאמר "לא תתעב אדומי" (דברים כג,ח).
Not to prevent the third generation of [converts from] the descendants of Esau from marrying among [the Jewish people], as [Deuteronomy 23:8] states: "Do not [utterly] despise an Edomite."