1
general::
The person just mentioned has appeared, as in Why, speak of the devil? there's Jeannie. This expression is a shortening of the older Speak of the devil and he's sure to appear, based on the superstition that pronouncing the devil's name will cause his arrival on the scene. The figurative use was already explained in James Kelly's Scottish Proverbs (1721).
American Heritage Idioms
2
general::
Phrase(s): Speak of the devil (and in he walks). [and] Talk of the devil (and he is sure to appear).
Prov. Talk about a certain person, and that person appears. (Used when someone appears whom you have just been talking about.) • Alan: I haven’t seen Bob for weeks. Jane: Look, here comes Bob right now. Alan: Well, talk of the devil. • Hi, there. We were just talking about you. Speak of the devil and in he walks.
McGrawhill's American Idioms And Phrasal Verbs