Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 167

שלא ייטמא כוהן גדול ואפילו לקרוביו, שנאמר "לאביו ולאימו, לא ייטמא" (ויקרא כא,יא).

For a High Priest not to become impure [through contact with any corpse], even [those of] his relatives, as [Leviticus 21:11] states: "He shall not become impure, [even] for his father and mother."