Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 360
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 360, with English translation by Maimonides (Rambam).
שלא ייקח סריס בת ישראל, שנאמר "לא יבוא פצוע דכא" (דברים כג,ב).
For a man incapable of procreation not to marry a natural born Jewess, as [Deuteronomy 23:2] states: "A man with crushed testicles... may not enter [God's congregation]."