Numbers 18:26-29

26“Moreover, you shall speak to the Levites and say to them, ‘When you take from the Israelites the tithe which I have given to you from them as your inheritance, then you shall present an offering from it to the Lord, a tithe of the tithe [paid by the people].

27Your offering shall be credited to you as the grain from the threshing floor or as the full produce from the wine vat.

28Likewise you shall also present an offering to the Lord from all your tithes, which you receive from the Israelites; and from it you shall give the Lord’s offering to Aaron the priest.

29Out of all your gifts, you shall present every offering due to the Lord, from all the best of it, even the sacred part from them.’