"The first of your grain...you shall give to him"—Deuteronomy 18:4.
We are commanded to separate from our harvest Terumah Gedolah [the "great tithe." This tithe is given to the priest.]
This biblical precept only applies in the Land of Israel.
"Speak to the Levites and tell them: When you take from the Children of Israel the tithe that I have given to you from them as your inheritance, then you shall offer up from it a gift for G‑d, a tenth of the tenth"—Numbers 18:26.
The Levites are commanded to separate a tenth from the tithe that they receive from the Israelites, and to give it to the priest. This tithe is called Maaser min Hamaaser or Terumat Maaser.
The Levites are commanded to give this tithe from the choicest of the produce.
