Numbers 18:21
"And, behold, I have given the children of Levi all the tenth in Israel for an inheritance, for their service which they serve, even the service of the tabernacle of the congregation."
Cross References (ranked by relevance)
And that we should bring the firstfruits of our dough, and our offerings, and the fruit of all manner of trees, of wine and of oil, unto the priests, to the chambers of the house of our God; and the tithes of our ground unto the Levites, that the same Levites might have the tithes in all the cities of our tillage.
And at that time were some appointed over the chambers for the treasures, for the offerings, for the firstfruits, and for the tithes, to gather into them out of the fields of the cities the portions of the law for the priests and Levites: for Judah rejoiced for the priests and for the Levites that waited.
They clave to their brethren, their nobles, and entered into a curse, and into an oath, to walk in God’s law, which was given by Moses the servant of God, and to observe and do all the commandments of the LORD our Lord, and his judgments and his statutes;
And brought in the offerings and the tithes and the dedicated things faithfully: over which Cononiah the Levite was ruler, and Shimei his brother was the next.
And I, behold, I have taken your brethren the Levites from among the children of Israel: to you they are given as a gift for the LORD, to do the service of the tabernacle of the congregation.
And verily they that are of the sons of Levi, who receive the office of the priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham:
Then brought all Judah the tithe of the corn and the new wine and the oil unto the treasuries.
And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.
Let him that is taught in the word communicate unto him that teacheth in all good things.