Deuteronomy 18:3-5

3And this shall be the priest’s due from the people, from them that offer a sacrifice, whether it be ox or sheep; and they shall give unto the priest the shoulder, and the two cheeks, and the maw. 4The firstfruit also of thy grain, of thy wine, and of thine oil, and the first of the fleece of thy sheep, shalt thou give him. 5For Yhwh thy God hath chosen him out of all thy tribes, to stand to minister in the name of Yhwh, him and his sons for ever.