Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 159
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 159, with English translation by Maimonides (Rambam).
שלא ייקח כוהן חללה, שנאמר "וחללה לא ייקחו" (ויקרא כא,ז).
For a priest not to marry a chalalah, as [Leviticus 21:7] states: "They shall not marry... a chalalah."