Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 160
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 160, with English translation by Maimonides (Rambam).
שלא ייקח כוהן גרושה, שנאמר "ואישה גרושה מאישה לא ייקחו" (ויקרא כא,ז).
For a priest not to marry a divorcee, as [Leviticus 21:7] states: "...nor may they marry a woman divorced from her husband."