Sanhedrin 6A

וכי תימא: פליגי רבנן עליה דרבן שמעון בן גמליאל, והאמר רבי אבהו: שנים שדנו ⁠– לדברי הכל אין דיניהם דין! גברא אגברא קא רמית?

And if you would say the Rabbis disagree with Rabban Shimon ben Gamliel with regard to the minimum number of judges necessary to adjudicate, but doesn’t R' Abbahu say: With regard to a court of two judges that adjudicated cases of monetary law of any type, which would include cases of admissions and loans, everyone agrees that their judgment is not a valid judgment, as a court with fewer than 3 judges is invalid? The Talmud rejects this question: Are you setting the statement of one man against the statement of another man? Although R' Abbahu asserts that all agree that two judges cannot issue a binding judgment, Shmuel differs. According to Shmuel, the Rabbis hold that the judgment of two judges is considered binding.

גופא, אמר רבי אבהו: שנים שדנו דיני ממונות ⁠– לדברי הכל אין דיניהם דין. איתיביה רבי אבא לרבי אבהו: דן את הדין וזיכה את החייב, וחייב את הזכאי, טימא את הטהור, טיהר את הטמא ⁠– מה שעשה עשוי, ומשלם מביתו.

§ Since it was mentioned incidentally, the Talmud discusses the matter itself: R' Abbahu says: With regard to a court of two judges that adjudicated cases of monetary law of any type, which would include cases of admissions and loans, everyone agrees that their judgment is not a valid judgment. R' Abba raised an objection to R' Abbahu from a Mishnah (Bekhorot 28b): If a single judge adjudicated a case of monetary law and erroneously exonerated the litigant who should have been deemed liable, or deemed liable the litigant who should have been deemed exempt, or if one issued a halakhic ruling by which he deemed ritually impure that which is actually pure, or deemed pure that which is impure, what he did is done, i.e., his ruling is binding. And the judge nevertheless pays from his house, i.e., from his own pocket, for the loss he has caused. Evidently, the ruling of even a single judge constitutes a valid judgment.