Deuteronomy 18:3-5

3And this shall be the priest's due from the people, from them that sacrifice a sacrifice, whether ox, or sheep: they shall give unto the priest the shoulder, and the jawbones, and the maw.

4The firstfruits [also] of thy corn, of thy new wine, and of thine oil, and the firstfruits of the shearing of thy sheep, shalt thou give him;

5for Jehovah thy 1God hath chosen him out of all thy tribes, that he may stand to serve in the name of Jehovah, he and his sons continually.