Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 166

שלא ייטמא כוהן הדיוט לשאר מתים, שנאמר "לנפש לא ייטמא בעמיו" (ויקרא כא,א).

For a common priest not to become impure through contact with a corpse [with the exception of the specific instances permitted by the Torah], as [Leviticus 21:1] states: "He shall not become impure [through contact with] the dead."