Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 74
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 74, with English translation by Maimonides (Rambam).
שלא יעבוד הזר במקדש, שנאמר "וְזָר לֹא יִקְרַב אֲלֵיכֶם" (במדבר יח, ד).
For a non-priest not to serve in the Temple, as [Numbers 18:4] states: "An unauthorized person shall not approach them."