Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 126
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 126, with English translation by Maimonides (Rambam).
להפריש תרומה גדולה לכהן, שנאמר: "רֵאשִׁית דְּגָנְךָ תִּתֵּן לוֹ" (דברים יח, ד).
To separate the greater terumah [and give it] to the priest, as [Deuteronomy 18:4] states: "Give him the first of your grain."