I have probably posted this in another thread, but it follows naturally on the preceding joke. It is very old:

On his wedding night, the groom is very well satisfied with the bride, but, perhaps as a result, he is immediately consumed with jealousy. After an hour of worrying that she will be unfaithful, he manages to get to sleep, and he has a dream. A fairy appears to him, and gives him a ring, saying, "So long as you keep this ring on your finger, your wife will always be faithful to you." So he puts it on, but right away, the magic ring begins jerking back & forth on his finger, as if it were trying to get off. Then he hears his wife say "That's not right!", and he wakes up, and -- you had better not imagine where his finger is.

I think Freud cites this somewhere.