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 Rosh Hashanah 21b.
כתיב (קהלת י״ב:י׳) בקש קהלת למצוא דברי חפץ, בקש קהלת לדון דינין שבלב שלא בעדים ושלא בהתראה, יצתה בת קול ואמרה לו וכתוב יושר דברי אמת על פי שנים עדים או שלשה עדים יומת המת.
English: It is written (Kohelet 12:10) “Kohelet sought to find words of delight.” Kohelet sought to judge cases according to the heart, without witnesses and without warning. A heavenly voice went forth and said to him, “And it is written: upright words of truth — on the testimony of two witnesses or three witnesses shall the dead be put to death.”
Even Solomon’s desire to judge by clear circumstantial evidence was rejected; capital cases require formal witnesses and warning.
Derash on the verse (Bavli) — not a formal named middah of the 13. The verse is cited as the definitive rejection of judgment by mere estimation or private knowledge.
Capital punishment may never be imposed on the basis of personal knowledge or strong circumstantial evidence alone; two formal witnesses are indispensable.