The 126th mitzvah is that we are commanded to separate terumah gedolah1 [from our crops].

The source of this commandment is G‑d's statement,2 "You must give him [the Kohen] the first portion of your grain, [wine and oil]."

This Biblical prohibition applies only in Eretz Yisroel.

The details of this mitzvah are explained in tractate Terumos.