Albert Barnes Commentary Numbers 18:21

"And unto the children of Levi, behold, I have given all the tithe in Israel for an inheritance, in return for their service which they serve, even the service of the tent of meeting." — Numbers 18:21 (ASV)

Abraham paid tithes to Melchizedek: Jacob had promised the tithe of all with which God blessed him if he returned in peace to his father’s house. But now, for the first time, the Lord’s tithes are assigned to the Levites for their support . The payment of tithes to them is recognized in Nehemiah 10:37, Nehemiah 12:44, and Tobit 1:7.