Positive commandment · Mitzvat Aseh #129 of 613
Levites to give a tithe from the tithe
להיות הלוים מפרישין מעשר מן המעשר שלקחו מישראל ונותנין אותו לכהנים
The verse · Bamidbar 18:26
וְאֶל־הַלְוִיִּם תְּדַבֵּר וְאָמַרְתָּ אֲלֵהֶם כִּי־תִקְחוּ מֵאֵת בְּנֵי־יִשְׂרָאֵל אֶת־הַמַּעֲשֵׂר אֲשֶׁר נָתַתִּי לָכֶם מֵאִתָּם בְּנַחֲלַתְכֶם וַהֲרֵמֹתֶם מִמֶּנּוּ תְּרוּמַת ה׳ מַעֲשֵׂר מִן־הַמַּעֲשֵׂר
“Thus speak to the Levites, and say to them, When you take of the children of Yisra᾽el the tithes which I have given you from them for your inheritance, then you shall offer up from it a gift for the Lord, even a tenth part of the tithe.”
The Rambam’s words
For the Levites to separate a tenth from the tenth which they took from the Israelites and give it to the priests, as [Numbers 18:27] states: "Speak to the Levites:...."
להיות הלוים מפרישין מעשר מן המעשר שלקחו מישראל ונותנין אותו לכהנים, שנאמר: "וְאֶל הַלְוִיִּם תְּדַבֵּר" (במדבר יח, כו).
Why · Sefer HaChinuch #396
Levites to give a tithe from the tithe
“From the roots of the commandment, [there is a need to] preface that there is no doubt that G-d separated the entire tribe of Levi from the congregation of Israel to always serve Him.”
Who & when
“This commandment is practiced in the place that the priestly tithe and the tithe are practiced.”
Where in the Mishneh Torah
Hilchot Heave Offerings הלכות תרומות