Exodus 28:42-43

Amplified Bible

⁴² You shall make for them [white] linen undergarments to cover their bare flesh, reaching from the waist to the thighs. ⁴³ The 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.