Positive commandment · Mitzvat Aseh #126 of 613
To separate the greater terumah [and give it] to the priest
להפריש תרומה גדולה לכהן
The verse · Devarim 18:4
רֵאשִׁית דְּגָנְךָ תִּירֹשְׁךָ וְיִצְהָרֶךָ וְרֵאשִׁית גֵּז צֹאנְךָ תִּתֶּן־לוֹ
“The firstfruit also of thy corn, of thy wine, and of thy oil, and the first of the fleece of thy sheep, shalt thou give him.”
The Rambam’s words
To separate the greater terumah [and give it] to the priest, as [Deuteronomy 18:4] states: "Give him the first of your grain."
להפריש תרומה גדולה לכהן, שנאמר: "רֵאשִׁית דְּגָנְךָ תִּתֵּן לוֹ" (דברים יח, ד).
Why · Sefer HaChinuch #507
To separate the great tithe for the priest
“It is from the roots of the commandment [that] since grain, wine and oil are the essence of people’s nourishment and the whole world is the Holy One’s, blessed be He, therefore it is fit for a man to remember his Creator for the blessing with which He blessed him, and to separate some of it for His sake, blessed be He, and give it to His servants — which are the priests that are constantly engaged in Heavenly service — before the hand of a man touch it and benefit from it at all.”
Who & when
“And outside the Land is also divided into two parts: the Lands of Egypt, Shinaar, Ammon and Moav, where priestly tithes and tithes are practiced rabbinically; and all of the other lands, where priestly tithes are not practiced at all.”
Where in the Mishneh Torah
Hilchot Heave Offerings הלכות תרומות