Numbers 18:21-24 New Revised Standard Version (NRSV)
21 To the Levites I have given every tithe in Israel for a possession in return for the service that they perform, the service in the tent of meeting. 22 From now on the Israelites shall no longer approach the tent of meeting, or else they will incur guilt and die. 23 But the Levites shall perform the service of the tent of meeting, and they shall bear responsibility for their own offenses; it shall be a perpetual statute throughout your generations. But among the Israelites they shall have no allotment, 24 because I have given to the Levites as their portion the tithe of the Israelites, which they set apart as an offering to the Lord. Therefore I have said of them that they shall have no allotment among the Israelites.
Deuteronomy 14:22-29 New Revised Standard Version (NRSV)
Regulations concerning Tithes
22 Set apart a tithe of all the yield of your seed that is brought in yearly from the field. 23 In the presence of the Lord your God, in the place that he will choose as a dwelling for his name, you shall eat the tithe of your grain, your wine, and your oil, as well as the firstlings of your herd and flock, so that you may learn to fear the Lord your God always. 24 But if, when the Lord your God has blessed you, the distance is so great that you are unable to transport it, because the place where the Lord your God will choose to set his name is too far away from you, 25 then you may turn it into money. With the money secure in hand, go to the place that the Lord your God will choose; 26 spend the money for whatever you wish—oxen, sheep, wine, strong drink, or whatever you desire. And you shall eat there in the presence of the Lord your God, you and your household rejoicing together. 27 As for the Levites resident in your towns, do not neglect them, because they have no allotment or inheritance with you.
28 Every third year you shall bring out the full tithe of your produce for that year, and store it within your towns; 29 the Levites, because they have no allotment or inheritance with you, as well as the resident aliens, the orphans, and the widows in your towns, may come and eat their fill so that the Lord your God may bless you in all the work that you undertake.