Exodus 28:39-43

And you shall weave the long and sleeved tunic of checkerwork of fine linen or silk and make a turban of fine linen or silk; and you shall make a girdle, the work of the embroiderer. For Aaron's sons you shall make long and sleeved tunics and belts or sashes and caps, for glory and honor and beauty. And you shall put them on Aaron your brother and his sons with him, and shall anoint them and ordain and sanctify them [set them apart for God], that they may serve Me as priests. You shall make for them [white] linen trunks to cover their naked flesh, reaching from the waist to the thighs. And they shall be on Aaron and his sons when they go into the Tent of Meeting or when they come near to the altar to minister in the Holy Place, lest they bring iniquity upon themselves and die; it shall be a statute forever to Aaron and to his descendants after him.