Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 127
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 127, with English translation by Maimonides (Rambam).
להפריש מעשר דגן ללוים, שנאמר: "כָּל מַעְשַׂר הָאָרֶץ" (ויקרא כז, ל).
To separate a tithe of grain [and give it] to the Levites, as [Leviticus 27:30] states: "All the land's tithes...."