Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 55
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 55, with English translation by Maimonides (Rambam).
שלא להרחיק מצרי מלבוא בקהל אלא עד שלושה דורות, שנאמר "לא תתעב מצרי" (דברים כג,ח).
Not to prevent the third generation of Egyptian [converts] from marrying among [the Jewish people], as [Deuteronomy 23:8] states: "Do not [utterly] despise an Egyptian."