Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 76

שלא יעבוד כהן טבול יום עד שיעריב שמשו, שנאמר "וְלֹא יְחַלְּלוּ" (ויקרא כא, ו).

For a priest who has immersed himself in a mikveh in order to purify himself from ritual impurity not to serve [in the Temple] until the conclusion, of that day, as [Leviticus 21:6] states: "And they shall not profane...."