Negative commandment · Lo Ta’aseh #136 of 613
For a priest who is impure not to partake of terumah
שלא יאכל כוהן טמא תרומה
The verse · Vayikra 22:4
אִישׁ אִישׁ מִזֶּרַע אַהֲרֹן וְהוּא צָרוּעַ אוֹ זָב בַּקֳּדָשִׁים לֹא יֹאכַל עַד אֲשֶׁר יִטְהָר וְהַנֹּגֵעַ בְּכׇל־טְמֵא־נֶפֶשׁ אוֹ אִישׁ אֲשֶׁר־תֵּצֵא מִמֶּנּוּ שִׁכְבַת־זָרַע
“What man soever of the seed of Aharon that is stricken with żara῾at, or has a running issue; he shall not eat of the holy things, until he be clean. And whoever touches anything that is unclean by the dead, or a man whose semen goes from him;”
The Rambam’s words
For a priest who is impure not to partake of terumah, as [Leviticus 22:4] states: "Any of your descendants... shall not eat from the consecrated foods."
שלא יאכל כוהן טמא תרומה, שנאמר "איש מזרעך . . . בקודשים לא יאכל" (ראה ויקרא כא,יז; ויקרא כב,ד).
Why · Sefer HaChinuch #279
That an impure priest not eat priestly tithe
“It is from the roots of the commandment [that it is] to aggrandize and make dear all that is [part of] the holy in the heart of every man. And I have already mentioned many times the benefit that comes out to us in the thing.”
Who & when
“And [it] is practiced by male and female priests in the Land and at the time that it is in its settlement — as then the obligation of the priestly tithe is from Torah writ there, according to the opinion of Rambam, may his memory be blessed (Mishneh Torah, Laws of Heave Offerings 1:26).”
Where in the Mishneh Torah
Hilchot Heave Offerings הלכות תרומות