Exodus 28:41-43

41And thou shalt clothe with them Aaron thy brother, and his sons with him; and shalt anoint them, and consecrate them, and hallow them, that they may serve me as priests. 42And thou shalt make them linen trousers to cover the flesh of nakedness; from the loins even to the hips shall they reach. 43And they shall be upon Aaron and his sons when they enter into the tent of meeting, or when they come near to the altar to serve in the sanctuary; that they may not bear iniquity and die — an everlasting statute for him and his seed after him.