The Sharia of Moses in the TORA
Part 2 - The 365 Prohibitions of the Tora
291 - A WITNESS AS AN ADVOCATE
By this prohibition, a witness is forbidden to act as an advocate in a case in which he gives evidence. Even though he be learned and well-informed, he may not act as witness, judge and advocate, but must testify concerning that which he has seen, and be silent, while the judges make such use of his testimony as they deem fit. The witness is forbidden to say anything in addition to his testimony. This prohibition applies only to capital cases.
In the Gemara of Sanhedrin we read: “one witness is not sufficient testimony against a person either for acquittal or for conviction”, and the following reason is given: “He has the appearance of an interested witness.”*
In cases involving monetary claims, a witness who has offered testimony may act as advocate, but he is excluded from presiding as judge.*