Exodus 28:41-43

41Put these on your brother Aaron and his sons; then anoint, ordain, and consecrate them, so that they may serve me as priests.

42Make them linen undergarments to cover their naked bodies; they must extend from the waist to the thighs.

43These must be worn by Aaron and his sons whenever they enter the tent of meeting or approach the altar to minister in the sanctuary area, so that they do not incur guilt and die. This is to be a permanent statute for Aaron and for his future descendants.