Negative commandment · Lo Ta’aseh #76 of 613
That a priest who immersed that day (tevul yom) not serve until his sun sets
שלא יעבוד כהן טבול יום עד שיעריב שמשו
The verse · Vayikra 21:6
קְדֹשִׁים יִהְיוּ לֵאלֹהֵיהֶם וְלֹא יְחַלְּלוּ שֵׁם אֱלֹהֵיהֶם כִּי אֶת־אִשֵּׁי ה׳ לֶחֶם אֱלֹהֵיהֶם הֵם מַקְרִיבִם וְהָיוּ קֹדֶשׁ
“They shall be holy to their God, and not profane the name of their God: for the offerings of the Lord made by fire, the bread of their God, they do offer: therefore they shall be holy.”
The Rambam’s words
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...."
שלא יעבוד כהן טבול יום עד שיעריב שמשו, שנאמר "וְלֹא יְחַלְּלוּ" (ויקרא כא, ו).
Why · Sefer HaChinuch #265
That a priest who immersed that day (tevul yom) not serve until his sun sets
“It is from the roots of the commandment [that it is] since the priest is the messenger between Israel and their Father in the heavens (based on Yoma 18b).”
Who & when
“And this commandment is practiced by male priests at the time of the [Temple] — since they are warned about the service and to be careful from impurity, and not priestesses.”
Where in the Mishneh Torah
Hilchot Admission into the Sanctuary הלכות ביאת מקדש