Exodus 28:41-43

41Clothe your brother, Aaron, and his sons with these garments, and then anoint and ordain them. Consecrate them so they can serve as my priests.

42Also make linen undergarments for them, to be worn next to their bodies, reaching from their hips to their thighs.

43These must be worn whenever Aaron and his sons enter the Tabernacle or approach the altar in the Holy Place to perform their priestly duties. Then they will not incur guilt and die. This is a permanent law for Aaron and all his descendants after him.