Negative commandment · Lo Ta’aseh #72 of 613
That the priests not be involved in the service of the Levites and the Levites in the service of the priests
שלא יתעסקו הלויים בעבודת הכהנים ולא הכהנים בעבודת הלויים
The verse · Bamidbar 18:3
וְשָׁמְרוּ מִשְׁמַרְתְּךָ וּמִשְׁמֶרֶת כׇּל־הָאֹהֶל אַךְ אֶל־כְּלֵי הַקֹּדֶשׁ וְאֶל־הַמִּזְבֵּחַ לֹא יִקְרָבוּ וְלֹא־יָמֻתוּ גַם־הֵם גַּם־אַתֶּם
“And they shall keep thy charge, and the charge of all the tent: only they shall not come near the vessels of the sanctuary and the altar, that neither they, nor you, die.”
The Rambam’s words
For the Levites not to perform the services of the priests, and for the priests not to perform the services of the Levites, as [Numbers 18:3] states with regard to the Levites: "Neither to the sacred vessels nor to the altar shall they approach, so that they do not die, neither they nor you."
שלא יתעסקו הלויים בעבודת הכהנים ולא הכהנים בעבודת הלויים, שנאמר "אֶל כְּלֵי הַקֹּדֶשׁ וְאֶל הַמִּזְבֵּחַ לֹא יִקְרְבוּ גַם הֵם גַּם אַתֶּם" (במדבר יח, ג).
Why · Sefer HaChinuch #389
That the priests not be involved in the service of the Levites and the Levites in the service of the priests
“It is from the roots of the commandment [that] because the service of these two groups is a very precious and holy service, therefore the work needs to be very guarded from abandon, from slothfulness and from forgetfulness.”
Who & when
“And it is practiced at the time of the [Temple] by priests and Levites.”
Where in the Mishneh Torah
Hilchot Vessels of the Sanctuary and Those Who Serve Therein הלכות כלי המקדש והעובדין בו