Numbers 18:26-29

26Moreover thou shalt speak unto the Levites, and say unto them, When ye take of the children of Israel the tithe which I have given you from them for your inheritance, then ye shall offer up an heave offering of it for the LORD, a tithe of the tithe.

27And your heave offering shall be reckoned unto you, as though it were the corn of the threshing-floor, and as the fulness of the winepress.

28Thus ye also shall offer an heave offering unto the LORD of all your tithes, which ye receive of the children of Israel; and thereof ye shall give the LORD’S heave offering to Aaron the priest.

29Out of all your gifts ye shall offer every heave offering of the LORD, of all the best thereof, even the hallowed part thereof out of it.