Exodus 28:41-43

41After you put these clothes on your brother Aaron and his sons, anoint and ordain them. Consecrate them so they may serve me as priests. 42“Make linen undergarments as a covering for the body, reaching from the waist to the 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 lasting ordinance for Aaron and his descendants.