LEV496 Someone who has information that can be helpful to a person on trial is obligated to come forward, whether or not he has been subpoenaed. To withhold testimony that can help save someone from undeserved punishment is likewise seen as standing idly by while your neighbor's blood [or reputation] is shed (Sifra on this verse]. In monetary cases, Maimonides applies this verse to one who withholds evidence that can prevent his fellow's money from being lost, but who does not come forward to testify (Book of the Commandments, negative commandment 297).