Exodus 28:27-28

27And you shall make two rings of gold and put them on the bottom of the two shoulder pieces of the ephod, on the front of it close to the place where it is joined, above the skillfully woven band of the ephod.

28And they shall bind the breastpiece by its rings to the rings of the ephod with a violet cord, so that it will be on the skillfully woven band of the ephod, and that the breastpiece will not come loose from the ephod.