Deuteronomy 18:3-5

This is what the priests get from the people from any offering of an ox or a sheep: the shoulder, the two cheeks, and the stomach. You must also give them the firstfruits of your grain, wine, and oil and the first fleece of your sheep, because GOD, your God, has chosen only them and their children out of all your tribes to be present and serve always in the name of GOD, your God.