Exodus 28:41-43

41After you put these garments on your brother Aaron and his sons, anoint them, ordain them, and consecrate them so that they may serve Me as priests.

42Make linen undergarments to cover their bare flesh, extending from waist to thigh. 43Aaron and his sons must wear them whenever they enter the Tent of Meeting or approach the altar to minister in the Holy Place, so that they will not incur guilt and die. This is to be a permanent statute for Aaron and his descendants.