Exodus 28:35-37

35And it shall be on Aaron when he ministers, and its sound shall be heard when he goes into the Holy Place before the Lord, and when he comes out, so that he does not die.

36"You shall make a plate of pure gold and engrave on it, like the engraving of a signet, 'Holy to the Lord.'

37And you shall fasten it on the turban by a cord of blue. It shall be on the front of the turban.