Exodus 28:41-43

41So you shall put them on Aaron your brother and on his sons with him. You shall anoint them, consecrate them, and sanctify them, that they may minister to Me as priests.

42And you shall make for them linen trousers to cover their nakedness; they shall reach from the waist to the thighs.

43They shall be on Aaron and on his sons when they come into the tabernacle of meeting, or when they come near the altar to minister in the holy place, that they do not incur iniquity and die. It shall be a statute forever to him and his descendants after him.