Hilchos Rechilus, chapter 6, section 10

Many people stumble here. If someone informed on them to the authorities and caused them a loss, and they have clear signs about who did it, they go and inform on him in return. This is forbidden for 2 reasons:

1. Informing on him is allowed only if it helps for the future — so that the other person won't inform on him again — but not to take revenge.

2. The whole permission is only if he saw himself that the person informed on him. Signs that he probably informed on him are not enough. And if he only heard it from others, it is surely forbidden to believe it, and surely forbidden to cause him a money loss.