Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 68
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 68, with English translation by Maimonides (Rambam).
שלא יכנס כהן להיכל בכל עת, שנאמר "וְאַל יָבֹא בְּכָל עֵת אֶל הַקֹדֶשׁ" (ויקרא טז, ב).
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."