Shemiras Halashon Halachos

Hilchos Rechilus, chapter 6, section 3

If a person's business suffered damage, one may not suspect a Jew of causing it [unless there is some reason to]. Even if one heard that a certain person caused the damage, one may only suspect it is true if this will help for the future. One may not believe it even if people fight with the suspected person and directly accuse him of doing the damage and he keeps silent.

Hilchos Rechilus, chapter 6, section 4

One may not believe rechilus even if one heard the story from two people or if there was a rumor in town, or if it is printed that so and so spoke or did something against one. This applies even if the intent of the publicizers was for one's benefit.