Exodus 28:41-43

41Put them on Aaron your brother and on his sons with him, and anoint them, consecrate them, and sanctify them, so that they may minister to Me as kohanim.

42“You are to make linen undergarments for them, to cover the skin of their nakedness, from the hips to the thighs.

43They are to be worn by Aaron and his sons when they go into the Tent of Meeting or when they approach the altar to minister in the holy place, so that they do not become subject to guilt and die. It is to be a statute forever, to him and to his offspring after him.