Negative commandment · Lo Ta’aseh #68 of 613
For a priest not to enter the Temple building at all times
שלא יכנס כהן להיכל בכל עת
The verse · Vayikra 16:2
וַיֹּאמֶר ה׳ אֶל־מֹשֶׁה דַּבֵּר אֶל־אַהֲרֹן אָחִיךָ וְאַל־יָבֹא בְכׇל־עֵת אֶל־הַקֹּדֶשׁ מִבֵּית לַפָּרֹכֶת אֶל־פְּנֵי הַכַּפֹּרֶת אֲשֶׁר עַל־הָאָרֹן וְלֹא יָמוּת כִּי בֶּעָנָן אֵרָאֶה עַל־הַכַּפֹּרֶת
“and the Lord said to Moshe, Speak to Aharon thy brother, that he come not at all times into the holy place within the veil before the covering, which is upon the ark; that he die not: for I appear in the cloud upon the ark cover.”
The Rambam’s words
For a priest not to enter the Temple building at all times, as [Leviticus 16:2] states: "And he shall not enter the sanctuary at all times."
שלא יכנס כהן להיכל בכל עת, שנאמר "וְאַל יָבֹא בְּכָל עֵת אֶל הַקֹדֶשׁ" (ויקרא טז, ב).
Why · Sefer HaChinuch #184
That the priests not enter at any time into the Temple — and all the more so, non-priests
“It is from the roots of the commandment that the servants fix the greatness of the place and its loftiness in their souls, and that its awe always be upon their faces.”
Who & when
“And [it] is practiced at the time of the [Temple] by the priests.”
Where in the Mishneh Torah
Hilchot Admission into the Sanctuary הלכות ביאת מקדש