Numbers 18:24-28

24For the tithe of the people of Israel, which they present as a contribution to the Lord, I have given to the Levites for an inheritance. Therefore I have said of them that they shall have no inheritance among the people of Israel."

25And the Lord spoke to Moses, saying,

26"Moreover, you shall speak and say to the Levites, 'When you take from the people of Israel the tithe that I have given you from them for your inheritance, then you shall present a contribution from it to the Lord, a tithe of the tithe.

27And your contribution shall be counted to you as though it were the grain of the threshing floor, and as the fullness of the winepress.

28So you shall also present a contribution to the Lord from all your tithes, which you receive from the people of Israel. And from it you shall give the Lord's contribution to Aaron the priest.