Numbers 18:25-29

And the LORD spoke to Moses, saying, “Moreover, you shall speak and say to the Levites, ‘When you take from the people of Israel the tithe that I have given you from them for your inheritance, then you shall present a contribution from it to the LORD, a tithe of the tithe. And your contribution shall be counted to you as though it were the grain of the threshing floor, and as the fullness of the winepress. So you shall also present a contribution to the LORD from all your tithes, which you receive from the people of Israel. And from it you shall give the LORD’s contribution to Aaron the priest. Out of all the gifts to you, you shall present every contribution due to the LORD; from each its best part is to be dedicated.’