Charles Ellicott Commentary Numbers 18:26

"Moreover 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 a heave-offering of it for Jehovah, a tithe of the tithe." — Numbers 18:26 (ASV)

Even a tenth part of the tithe.— Better, a tithe of the tithe.