Deuteronomy 19:15

19:15 A single witness may not testify against another person for any trespass or sin that he commits. A matter may be legally established only on the testimony of two or three witnesses.

Deuteronomy 19:18-19

19:18 The judges will thoroughly investigate the matter, and if the witness should prove to be false and to have given false testimony against the accused, 19:19 you must do to him what he had intended to do to the accused. In this way you will purge evil from among you.