Here a "matter" is an accusation and "mouth" is a metonym for the testimony a person gives through his mouth. This can be stated in active form. Alternate translation: "Believe that someone has done something wrong only after two or three people have given the same testimony" (See: [[rc://en/ta/man/jit/figs-activepassive]] and [[rc://en/ta/man/jit/figs-metonymy]])