Rosh Hashanah 22B

וחד מי מהימן?! והתניא: מעשה שבא הוא ועדיו עמו להעיד עליו! אמר רב פפא: מאי ״אחר״⁠ — זוג אחר.

but is one witness deemed credible? Isn’t it taught in a baraita: There was an incident in which one potential witness came to testify, and his witnesses were with him, as they came to testify about him? The use of the plural indicates that two witnesses are required to establish someone as a valid eyewitness. Rav Pappa said: What is the meaning of the term: Another? It means another pair of witnesses.

הכי נמי מסתברא, דאי לא תימא הכי ⁠— ״אם אינן מכירין אותו״, מאי ״אותו״? אילימא ״אותו״ חד ⁠— וחד מי מהימן? ״משפט״ כתיב ביה. אלא מאי ״אותו״⁠ — אותו הזוג, הכי נמי: מאי ״אחר״⁠ — זוג אחר.

The Talmud comments: This too stands to reason, for if you do not say so, then the opening statement of the Mishnah: If the members of the Great Sanhedrin are not familiar with that one, is problematic. What is the meaning of the term: That one? If we say it is referring to that one witness, is one witness deemed credible? The word: Judgment, is written with regard to the establishment of Rosh Chodesh and Rosh HaShana: “For it is a statute for Israel, a judgment of the God of Israel” (Psalms 81:5), and judgments require two witnesses. Rather, what is the meaning of the term: That one? That pair of witnesses. So too here, what is the meaning of the term: Another? Another pair of witnesses.

וחד לא מהימן? והתניא: מעשה ברבי נהוראי שהלך אצל העד להעיד עליו בשבת באושא!

The Talmud asks: And is one witness not deemed credible to testify about the eyewitness who saw Rosh Chodesh? Isn’t it taught in a baraita: There was an incident involving R' Nehorai, who went with the witness to testify about him on Shabbat in Usha? Apparently, R' Nehorai offered his testimony alone.