Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 23
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 23, with English translation by Maimonides (Rambam).
לִהְיוֹת הַלֵּוִי עוֹבֵד בַּמִּקְדָּשׁ, שֶׁנֶּאֱמָר: "וְעָבַד הַלֵּוִי הוּא" (במדבר יח, כג).
For the Levites to serve in the sanctuary, as [Numbers 18:23] states: "And the Levite shall serve...."