Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 158
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 158, with English translation by Maimonides (Rambam).
שלא ייקח כוהן זונה, שנאמר "אישה זונה וחללה לא ייקחו" (ויקרא כא,ז).
For a priest not to marry an immoral woman [zonah], as [Leviticus 21:7] states: "They shall not marry an immoral woman or a chalalah."