Numbers 18:25-29

Then the LORD spoke to Moses, saying, “Speak thus to the Levites, and say to them: ‘When you take from the children of Israel the tithes which I have given you from them as your inheritance, then you shall offer up a heave offering of it to the LORD, a tenth of the tithe. And your heave offering shall be reckoned to you as though it were the grain of the threshing floor and as the fullness of the winepress. Thus you shall also offer a heave offering to the LORD from all your tithes which you receive from the children of Israel, and you shall give the LORD’s heave offering from it to Aaron the priest. Of all your gifts you shall offer up every heave offering due to the LORD, from all the best of them, the consecrated part of them.’