Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 183
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 183, with English translation by Maimonides (Rambam).
לתת ללויים ערים לשבת, וגם הן קולטות, שנאמר: "ונתנו ללויים . . . ערים" (במדבר לה,ב).
To give the Levites cities in which to dwell - they also serve as refuge centers - as [Numbers 35:2] states: "And you shall give cities to the Levites...."