This true story is one of the many told by St. Louis de Montfort in his book The Secret of the Rosary.
Alphonsus, King of Leon and Galicia (the northwest corner of present day Spain), very much wanted all his servants to honor the Blessed Virgin by saying the Rosary. So he used to hang a large rosary on his belt and always wore it, but unfortunately never said it himself. Nevertheless, his wearing it encouraged his courtiers to say the Rosary very devoutly.
One day the King fell seriously ill and when he was given up for dead he found himself, in a vision, before the judgment seat of Our Lord. Many devils were there accusing him of all the sins he had committed. Our Lord, as Sovereign Judge, was just about to condemn him to hell, when Our Lady appeared, to intercede for the king. She called for a pair of scales and had his sins placed in one of the balances, whereas She put the Rosary that he had always worn on the other scale, together with all the Rosaries that had been said because of his example. It was found that Rosaries weighed more than his sins.