Yevamot 88A

מאי טעמא קא מחייבי רבנן? אילימא משום דמהימן ⁠— והא תרי בעלמא, דאף על גב דקא מכחיש להו אינהו ⁠— מהימני, וקא פטרי רבנן. אלא לאו, משום דאישתיק ⁠— ושתיקה כהודאה דמיא.

of the Mishnah, what is the reason that when he remains silent, the Rabbis obligate him to bring to an offering based on the testimony of one witness? If we say it is because the witness is deemed credible, but there is the case of an ordinary pair of witnesses, where even though he contradicts their claim they are deemed credible, and yet the Rabbis exempt him from bringing an offering. If so, they would certainly not obligate him to bring an offering due to the testimony of a lone witness. Rather, is it not because he remained silent, and silence is considered like an admission? If this is the reason why he brings an offering, there is no proof from here that the testimony of one witness is accepted.

אלא סברא היא, מידי דהוה אחתיכה ספק של חלב ספק של שומן, ואתא עד אחד ואמר: ברי לי, דשומן הוא ⁠— דמהימן.

Rather, this is evidently based on logical reasoning: Just as it is in the case of a piece of meat about which it is uncertain if it is forbidden fat and uncertain if it is of permitted fat, and there is no way of clarifying which it is, and one witness comes and says: It is clear to me that it is permitted fat, the halakha is that he is deemed credible. Here too, the testimony of a single witness can resolve the uncertainty.

מי דמי? התם לא איתחזק איסורא, הכא איתחזק איסורא דאשת איש, ואין דבר שבערוה פחות משנים!

The Talmud raises a difficulty: Is it comparable? There, the presumption of a prohibition has not been established, as there is no proof that the piece was ever forbidden, and one can therefore rely on the witness who permits it, whereas here, the presumption of the prohibition with regard to a married woman is established, and there is a principle that nothing involving those with whom sex are forbidden can be determined by fewer than two witnesses.