Numbers 18:25-29

The LORD also told Moses, “Give these instructions to the Levites: When you receive from the people of Israel the tithes I have assigned as your allotment, give a tenth of the tithes you receive—a tithe of the tithe—to the LORD as a sacred offering. The LORD will consider this offering to be your harvest offering, as though it were the first grain from your own threshing floor or wine from your own winepress. You must present one-tenth of the tithe received from the Israelites as a sacred offering to the LORD. This is the LORD’s sacred portion, and you must present it to Aaron the priest. Be sure to give to the LORD the best portions of the gifts given to you.