Exodus 28:35-37

35Aaron must wear it when he ministers. The sound of the bells will be heard when he enters the Holy Place before the Lord and when he comes out, so that he will not die.

36“Make a plate of pure gold and engrave on it as on a seal: holy to the Lord.

37Fasten a blue cord to it to attach it to the turban; it is to be on the front of the turban.