Numbers 18:26-29 New Revised Standard Version (NRSV)
26 You shall speak to the Levites, saying: When you receive from the Israelites the tithe that I have given you from them for your portion, you shall set apart an offering from it to the Lord, a tithe of the tithe. 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.