עַל־פִּ֣י שְׁנַ֣יִם עֵדִ֗ים א֛וֹ שְׁלֹשָׁ֥ה עֵדִ֖ים יוּמַ֣ת הַמֵּ֑ת לֹ֣א יוּמַ֔ת עַל־פִּ֖י עֵ֥ד אֶחָֽד׃
A person shall be put to death only on the testimony of two or more witnesses; no one shall be put to death on the testimony of a single witness.—
Torah Temimah points to Sanhedrin (the incident of Shimon ben Shetach).
תניא, א״ר שמעון בן שטח, אראה בנחמה אם לא ראיתי אחד שרץ אחר חבירו לחורבה ורצתי אחריו וראיתי סייף בידו ודם מטפטף והרוג מפרפר, ואמרתי לו, רשע, מי הרגו לזה, אני או אתה, אבל מה אעשה שאין דמך מסור בידי, שהרי אמרה תורה על פי שנים עדים יומת המת, היודע מחשבות יפרע מאותו האיש שהרג את חבירו.
English: It was taught: Rabbi Shimon ben Shetach said, May I see consolation if I did not once see a man running after his fellow into a ruin. I ran after him and saw a sword in his hand and blood dripping, and the slain man convulsing. I said to him, “Wicked one, who killed this man — I or you? But what can I do, for your blood is not delivered into my hand, for the Torah said ‘on the testimony of two witnesses shall the dead be put to death.’ He who knows thoughts will exact payment from that man who killed his fellow.”
Even the clearest circumstantial evidence is insufficient for a capital verdict; formal testimony of two witnesses remains indispensable.
Derash on the verse (Bavli narrative) — not a formal named middah of the 13. The incident illustrates the absolute requirement of two witnesses.
Even when a judge personally witnesses near-certain evidence of murder, he may not condemn the perpetrator without two formal witnesses.