Exodus 28:42-43

42You shall make for them [white] linen undergarments to cover their bare flesh, reaching from the waist to the thighs.

43The various articles of clothing shall be on Aaron and on his sons when they enter the Tent of Meeting, or when they approach the altar [of incense] to minister in the Holy Place, so that they do not incur guilt and die. It shall be a statute forever to Aaron and to his descendants after him.