Sanhedrin 34A

מאי טעמא דרבי יוסי בר יהודה? דאמר קרא: ״עד אחד לא יענה בנפש למות״. למות הוא דאינו עונה, אבל לזכות עונה. ורבנן? אמר ריש לקיש: משום דמיחזי כנוגע בעדותו.

The Talmud explains: What is the reason of R' Yosei, son of R' Yehuda? As the verse states: “But one witness should not testify against any person that he die.” One can infer: That he die is the matter concerning which he does not testify, but he does testify to acquit. The Talmud asks: And what is the reasoning of the Rabbis? Reish Lakish says: A witness cannot offer any statements beyond his testimony because it appears as though he is biased in his testimony. If the court finds the accused liable based on the witness’s testimony, the witness could later be accused of being a conspiring witness. Therefore, it is to his advantage to have the court acquit the accused.

ורבנן, האי ״למות״ מאי דרשי ביה? מוקמי ליה באחד מן התלמידים. כדתניא: אמר אחד מן העדים ״יש לי ללמד עליו זכות״, מניין שאין שומעין לו? תלמוד לומר: ״עד אחד לא יענה״. מניין לאחד מן התלמידים שאמר ״יש לי ללמד עליו חובה״, מניין שאין שומעין לו? תלמוד לומר: ״אחד לא יענה בנפש למות״.

The Talmud asks: And as for the Rabbis, in what manner do they interpret this term: “That he die”? The Talmud answers: They establish it as teaching the halakha with regard to one of the students. As it is taught in a baraita: If one of the witnesses said: I have the ability to teach a reason to acquit him, from where is it derived that the court does not listen to him? The verse states: “One witness should not testify.” From where is it derived that if there is one of the students who said: I have the ability to teach a reason to deem him liable, from where is it derived that the court does not listen to him? The verse states: “But one witness should not testify against any person that he die.”