Hilchos Lashon Hara chapter 7, section 14

This shows how people err when they suspect someone of stealing something. They go to the town elders and say there are devarim nikarim that so and so was the thief, and the elders strike him until he confesses. This is wrong because even if devarim nikarim was as potent as seeing the actual theft and even if the elders were considered like a beis din, they would need to know themselves that he stole. Regarding devarim hanikarim, they would have to know them through witnesses or see them themselves (as in the incident of Mar Zutra). They cannot rely on the plaintiff and strike a Jew for no reason. They may not even believe the plaintiff in their hearts and they certainly may not rely on what he said to strike the person.