Positive commandment · Mitzvat Aseh #127 of 613
To separate a tithe of grain [and give it] to the Levites
להפריש מעשר דגן ללוים
The verse · Vayikra 27:30
וְכׇל־מַעְשַׂר הָאָרֶץ מִזֶּרַע הָאָרֶץ מִפְּרִי הָעֵץ לַה׳ הוּא קֹדֶשׁ לַה׳
“And all the tithe of the land, whether of the seed of the land, or of the fruit of the tree, is the Lord’s: it is holy to the Lord.”
The Rambam’s words
To separate a tithe of grain [and give it] to the Levites, as [Leviticus 27:30] states: "All the land's tithes...."
להפריש מעשר דגן ללוים, שנאמר: "כָּל מַעְשַׂר הָאָרֶץ" (ויקרא כז, ל).
Why · Sefer HaChinuch #395
First tithe
“It is from the roots of the commandment [that] since G-d, may He be blessed, chose the tribe of Levi from among his brothers for His service in the Temple always, therefore it was from His kindness to them to give them their sustenance in an honorable way.”
Who & when
“And it is practiced by males and females, by Israelites and by priests and by Levites.”
Where in the Mishneh Torah
Hilchot Second Tithes and Fourth Year's Fruit הלכות מעשר שני ונטע רבעי