D&C 17
When Joseph and Oliver learned of the prophecy that there would be Three Witnesses, Oliver, Martin, and David recognized (perhaps not consciously) the promises they had received before that they might learn anything if they had faith and they each asked if they might be one of the three. In this revelation they are told that they may if they rely upon His word with full purpose of heart. They are promised that they would see not only the plates, but the sword of Laban, the Urim and Thummim, and the Liahona as well, and that they would have power to testify as Joseph did to the truthfulness of the Book of Mormon.
Lest I forget later, Urim means “lights” and Thummim means “perfections”