Exodus 28:42-43

⁴²You shall make for them linen breeches to cover their bare flesh; they shall reach from the loins even to the thighs. ⁴³They shall be on Aaron and on his sons when they enter the tent of meeting, or when they approach the altar to minister in the holy place, so that they do not incur guilt and die. It shall be a statute forever to him and to his descendants after him.