Numbers 18:24-26

24For I have given them the tenth that the Israelites present to the Lord as a contribution for their inheritance. That is why I told them that they would not receive an inheritance among the Israelites.”

25The Lord instructed Moses,

26“Speak to the Levites and tell them: When you receive from the Israelites the tenth that I have given you as your inheritance, you are to present part of it as an offering to the Lord  — a tenth of the tenth.