Shevuot 33B

שהוציא איש פלוני שם רע על בתי״⁠ – חייבין. הודה מפי עצמו ⁠– פטור.

that so-and-so slandered my daughter. In such a case, the witnesses are liable for taking a false oath of testimony. If the accused admitted on his own that he slandered the young woman, he is exempt from paying the fine. The only payment in the case of a husband who falsely accuses his bride of adultery is payment of a fine, and yet the baraita says that the witnesses are liable for taking a false oath of testimony.

הא מני ⁠– רבי אלעזר ברבי שמעון היא, דאמר: יבואו עדים ויעידו.

The Talmud answers: In accordance with whose opinion is this baraita? It is in accordance with the opinion of R' Elazar, son of R' Shimon, who says: Even if the one who is liable to pay the fine admits his liability, the witnesses should come and testify and render the one who performed the action liable to pay the fine.

אימא סיפא: הודה מעצמו ⁠– פטור, אתאן לרבנן!

The Talmud asks: Say the latter clause of the baraita: If that man admitted on his own that he slandered the young woman, he is exempt from paying the fine. Ostensibly, in this we arrive at the opinion of the Rabbis, who hold that if the one who is liable admits his liability before the witnesses testify, he is exempt from paying the fine, contrary to the opinion of R' Elazar, son of R' Shimon.

כולה רבי אלעזר ברבי שמעון היא, והכי קאמר: לא משכחת לה הודה מפי עצמו דפטור, אלא היכא דליכא עדים כלל והודה מעצמו.

The Talmud explains: The baraita in its entirety is the opinion of R' Elazar, son of R' Shimon, and this is what the tanna is saying in the latter clause: You find a case of one who admitted liability on his own that he is exempt from paying the fine, only in a case where there are no witnesses at all and he admitted liability on his own. In a case where there are witnesses, he is liable to pay the fine on the basis of their testimony even if he admitted liability.