Numbers 18:26-29

²⁶Thou shalt also speak to the Levites, and shalt say to them, If ye take the tithe from the children of Israel, which I have given you from them for an inheritance, then shall ye separate from it a heave-offering to the Lord, a tenth of the tenth.

²⁷And your heave-offerings shall be reckoned to you as corn from the floor, and an offering from the wine-press.

²⁸So shall ye also separate them from all the offerings of the Lord out of all your tithes, whatsoever ye shall receive from the children of Israel; and ye shall give of them an offering to the Lord to Aaron the priest.

²⁹Of all your gifts ye shall offer an offering to the Lord, and of every first-fruit the consecrated part from it.