Exodus 28:36-38

36“You shall also make a plate of pure gold and engrave on it, like the engravings of a signet, ‘Holy to the Lord.’

37You shall fasten it on the front of the turban with a blue cord.

38It shall be on Aaron’s forehead, and Aaron shall take away the guilt from the holy things which the sons of Israel dedicate, with regard to all their holy gifts. It shall always be on his forehead, so that they may be accepted before the Lord.