Numbers 18:27-32

27 It shall be reckoned to you as your gift, the same as the grain of the threshing floor and the fullness of the wine press.
28 Thus you also shall set apart an offering to the Lord from all the tithes that you receive from the Israelites; and from them you shall give the Lord's offering to the priest Aaron.
29 Out of all the gifts to you, you shall set apart every offering due to the Lord; the best of all of them is the part to be consecrated.
30 Say also to them: When you have set apart the best of it, then the rest shall be reckoned to the Levites as produce of the threshing floor, and as produce of the wine press.
31 You may eat it in any place, you and your households; for it is your payment for your service in the tent of meeting.
32 You shall incur no guilt by reason of it, when you have offered the best of it. But you shall not profane the holy gifts of the Israelites, on pain of death.