Numbers 18:26-29 (New International Version)

²⁶ "Speak to the Levites and say to them: 'When you receive from the Israelites the tithe I give you as your inheritance, you must present a tenth of that tithe as the LORD's offering. ²⁷ Your offering will be reckoned to you as grain from the threshing floor or juice from the winepress. ²⁸ In this way you also will present an offering to the LORD from all the tithes you receive from the Israelites. From these tithes you must give the LORD's portion to Aaron the priest. ²⁹ You must present as the LORD's portion the best and holiest part of everything given to you.'