Numbers 18:24-26

24for I have given for an inheritance to the Levites the tithes of the children of Israel, which they offer as a heave-offering to Jehovah; therefore I have said of them, They shall possess no inheritance among the children of Israel.

25And Jehovah spoke to Moses, saying,

26And to the Levites shalt thou speak, and say unto them, When ye take of the children of Israel the tithes which I have given you from them for your inheritance, then ye shall offer a heave-offering from it for Jehovah, the tenth of the tithe.