Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 136
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 136, with English translation by Maimonides (Rambam).
שלא יאכל כוהן טמא תרומה, שנאמר "איש מזרעך . . . בקודשים לא יאכל" (ראה ויקרא כא,יז; ויקרא כב,ד).
For a priest who is impure not to partake of terumah, as [Leviticus 22:4] states: "Any of your descendants... shall not eat from the consecrated foods."