Exodus 28:30

28:30 “You are to put the Urim and the Thummim into the breastpiece of decision; and they are to be over Aaron’s heart when he goes in before the Lord. Aaron is to bear the decisions of the Israelites over his heart before the Lord continually.

Exodus 28:43

28:43 These must be on Aaron and his sons when they enter to the tent of meeting, or when they approach the altar to minister in the Holy Place, so that they bear no iniquity and die. It is to be a perpetual ordinance for him and for his descendants after him.