Numbers 18:24-28

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.

27Your offering will be credited to you as if it were your grain from the threshing floor or the full harvest from the winepress.

28You are to present an offering to the Lord from every tenth you receive from the Israelites. Give some of it to the priest Aaron as an offering to the Lord.