Negative commandment · Lo Ta’aseh #75 of 613
For a priest who is ritually impure not to serve [in the Temple]
שלא יעבוד כהן טמא
The verse · Vayikra 22:2
דַּבֵּר אֶל־אַהֲרֹן וְאֶל־בָּנָיו וְיִנָּזְרוּ מִקׇּדְשֵׁי בְנֵי־יִשְׂרָאֵל וְלֹא יְחַלְּלוּ אֶת־שֵׁם קׇדְשִׁי אֲשֶׁר הֵם מַקְדִּשִׁים לִי אֲנִי ה׳
“Speak to Aharon and to his sons, that they separate themselves from the holy things of the children of Yisra᾽el which they hallow to me, and they profane not my holy name, I am the Lord.”
The Rambam’s words
For a priest who is ritually impure not to serve [in the Temple], as [Leviticus 22:2] states: "And they shall separate themselves from the sacred offerings of the children of Israel."
שלא יעבוד כהן טמא, שנאמר "וְיִנָּזְרוּ מִקָּדְשֵׁי בְנֵי יִשְׂרָאֵל" (ויקרא כב, ב).
Who & when
“And [it] is practiced by priests at the time of the [Temple].”
Where in the Mishneh Torah
Hilchot Admission into the Sanctuary הלכות ביאת מקדש