Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 75

שלא יעבוד כהן טמא, שנאמר "וְיִנָּזְרוּ מִקָּדְשֵׁי בְנֵי יִשְׂרָאֵל" (ויקרא כב, ב).

For a priest who is ritually impure not to serve [in the Temple], as [Leviticus 22:2] states: "And they shall separate themselves from the sacred offerings of the children of Israel."