וְשָׁמְר֣וּ אֶת־מִשְׁמַרְתִּ֗י וְלֹֽא־יִשְׂא֤וּ עָלָיו֙ חֵ֔טְא וּמֵ֥תוּ ב֖וֹ כִּ֣י יְחַלְּלֻ֑הוּ אֲנִ֥י יְהוָ֖ה מְקַדְּשָֽׁם׃
They shall therefore keep My charge, lest they bear sin for it, and die therein, if they profane it: I am the LORD who sanctify them.
Rashi on Leviticus
ושמרו את משמרתי THEY SHALL THEREFORE KEEP MY CHARGE — not to eat the heave-offering (as mentioned in v. 7) whilst they are in a state of bodily uncleanness (Sanhedrin 83a).
Sforno on Leviticus
אני ה' מקדשם, I have sanctified them (the items sanctified as donations by the ordinary Israelites) It is therefore appropriate that priests who treat these sanctified objects with disdain (as if they were profane), be punished for desecrating them.
Siftei Chakhamim
Not to eat. Explanation: “They shall keep” is referring to [what is written] earlier where it was discussing terumah, as Rashi explained above.