Numbers 18:26-29

²⁶Speak also to the Levites, and say to them, When you take from the sons of Israel the tenths which I have given you of theirs, you shall set apart from them a separation before the Lord, a tenth of the tenth.

²⁷And your separation shall be reckoned to you as the corn from the threshing-floor, and as the wine of the wine-press;

²⁸So shall you set apart the separation before the Lord of all your tenths that you receive from the sons of Israel, and give thereof a separation before the Lord unto Aharon the priest.

²⁹Of all that is given to you ye shall set apart every separation for the Lord, of all the best of it, to be the hallowed portion thereof.