Numbers 18:26-29

²⁶Command the Levites, and declare unto them: When you shall receive of the children of Israel the tithes, which I have given you, offer the firstfruits of them to the Lord, that is to say, the tenth part of the tenth:

²⁷That it may be reckoned to you as an oblation of firstfruits, as well of the barnfloors as of the winepresses:

²⁸And of all the things of which you receive tithes, offer the firstfruits to the Lord, and give them to Aaron the priest.

²⁹All the things that you shall offer of the tithes, and shall separate for the gifts of the Lord, shall be the best and choicest things.