And their prophet said to them, "Indeed, a sign of his kingship is that the chest will come to you in which is assurance1 from your Lord and a remnant of what the family of Moses and the family of Aaron had left, carried by the angels. Indeed in that is a sign for you, if you are believers."
Their prophet said to them that the mark of his being truly chosen as their king was that Allah would return to them the Ark. This was a chest revered by the Israelites which was taken away from them. In it there was tranquillity and relics left behind by the family of Moses and Aaron, such as the staff and a few tablets. In that there is a clear sign if you are true believers.