Halakhah for Sanhedrin 53:17

Not to have a sinner testify: That we not accept the testimony of a man who sins, and we not do anything on account of his testimony, as it is stated (Exodus 23:1), "do not place your hand with an evildoer to be a violent witness." And the explanation comes about this (Sanhedrin 27a), "Do not place an evildoer as a witness, do not place a violent one as a witness," meaning a violent person - to exclude violent people and thieves who are disqualified from testimony, as it is stated (Deuteronomy 19:16), "If a violent witness comes upon a man."