Rashi on Numbers 18:28/רש"י על במדבר י״ח:כ״ח

1כן תרימו גם אתם THUS YE ALSO SHALL HEAVE AN HEAVE OFFERING just as the ordinary Israelites set apart a heave offering, from the produce of their threshing floors and wine presses, so you Levites, also, shall set apart a heave offering from your tithes, for that is your inheritance (cf. v. 24).