Exodus 28:42-43

28:42 Make for them linen undergarments to cover their naked bodies; they must cover from the waist to the thighs. 28:43 These must be on Aaron and his sons when they enter to the tent of meeting, or when they approach the altar to minister in the Holy Place, so that they bear no iniquity and die. It is to be a perpetual ordinance for him and for his descendants after him.