Numbers 18:26-29

26“You are to speak to the Levites, and you must tell them, ‘When you receive from the Israelites the tithe that I have given you from them as your inheritance, then you are to offer up from it as a raised offering to the Lord a tenth of the tithe.

27And your raised offering will be credited to you as though it were grain from the threshing floor or as new wine from the winepress.

28Thus you are to offer up a raised offering to the Lord of all your tithes which you receive from the Israelites; and you must give the Lord’s raised offering from it to Aaron the priest.

29From all your gifts you must offer up every raised offering due the Lord, from all the best of it, and the holiest part of it.’