Negative Commandment #75
Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
Second Section – the Negative Commandments
75. It is a negative commandment for a court "do not accept the testimony of a sinner"
As it states in Shemos (Exodus) 23:1, "do not give your hand to a wicked person to be a malicious witness." If a witness that is fit knows that a person is wicked and the judges do not know about this, it is forbidden for the fit witness to testify along with the wicked man, even if it is a truthful testimony. The reason is because of the commandment, "do not give your hand to the wicked." There is no need to state that he knows the second witness will give false testimony, because it is forbidden to be a witness with him.
This applies in all places and at all times.