Exodus 28:41-43

41You shall put them on Aaron your brother and on his sons with him; and you shall 1anoint them and 2ordain them and consecrate them, that they may serve Me as priests.

42You shall make for them 3linen breeches to cover their bare flesh; they shall 4reach from the loins even to the thighs.

43They shall be on Aaron and on his sons when they enter the tent of meeting, or 5when they approach the altar to minister in the holy place, so that they do not incur 6guilt and die. 7It shall be a statute forever to him and to his 8descendants after him.