There are sixteen possible sources for Much Ado about Nothing. Most of these sources relate only to the Hero and Claudio plot. In other versions of the Hero-Claudio story, a rival to Claudio causes the deception. Shakespeare, though, used Don John instead since the Hero and Claudio story is not the primary focus of his play.
The most notable of Shakespeare's sources are listed here:
Ariosto's epic includes an incident of a man in love who thinks that he sees his girl at her bedroom window with another lover.
This short book contains the same incident that appears in Ariosto's epic.