Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 354
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 354, with English translation by Maimonides (Rambam).
שלא יישא ממזר בת ישראל, שנאמר "לא יבוא ממזר, בקהל ה'" (דברים כג,ג).
For a mamzer not to marry a natural born Jewess, as [Deuteronomy 23:3 states: "A mamzer may not enter God's congregation."