Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 165
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 165, with English translation by Maimonides (Rambam).
שלא ייצא כוהן מן העזרה בשעת עבודה, שנאמר "ומפתח אוהל מועד לא תצאו" (ויקרא י,ז).
For a priest not to leave the Temple courtyard in the midst of service, as [implied by Leviticus 10:7]: "Do not depart from before the entrance to the Tent of Testimony."