This is an old German one.

A man had scarcely been married when he started being jealous of his wife. Even lying next to her in bed, he couldn't sleep for worrying that she might be unfaithful. At length, he fell into a fitful sleep, and dreamed that a fairy came to him and gave him a ring. Wear this ring, said the fairy, never take it off, and you can be sure that your wife will be faithful to you. So he put the ring on; but immediately, it started jerking on his finger as if it were trying to get off. He struggled to keep it on, and finally he awoke, and -- you had better not imagine where his finger was!