Exodus 39:21-23

Complete Jewish Bible

²¹ Then they bound the breastplate by its rings to the rings of the vest with a blue cord, so that it could be on the vest’s decorated belt, and so that the breastplate would not swing loose from the vest — as Adonai had ordered Moshe.

(RY: vi, LY: iii) ²² He made the robe for the ritual vest; it was woven entirely of blue, ²³ with its opening in the middle, like that of a coat of mail, and with a border around the opening, so that it wouldn’t tear.