Exodus 28:42-43

42You shall make for them 1linen undergarments to cover their bare flesh; they shall 2reach from the waist even to the thighs.

43And they shall be on Aaron and on his sons when they enter the tent of meeting, or 3when they approach the altar to minister in the Holy Place, so that they do not incur 4guilt and die. 5It shall be a statute forever to him and to his 6descendants after him.