Numbers 18:26

Thus speak unto the Levites and say unto them, When ye take of the children of Israel the tithes which I have given you from them for your inheritance, which was to be their regular income, upon which they should be able to depend, then ye shall offer up an heave-offering of it for the Lord, even a tenth part of the tithe. So the Levites also were obliged to set aside ten per cent. of their income for the Lord, this amount being given to the priests.