Gemara

Kidushin 65b (Abaye): If Reuven told Shimon 'you ate Chelev' your Taharos (Tahor food or Kelim) became Tamei', or 'a man had Bi'ah with your ox (so it is Pasul for a Korban)' and Shimon was silent, Reuven is believed.

A Mishnah and Beraisos support these teachings.

Gitin 54b (Beraisa): If Reuven was working with Shimon's Taharos and he told Shimon that they became Tamei, he is believed. If he tells him 'your Taharos that I worked with on a previous day became Tamei, he is not believed.

(Abaye): He is believed only in the Reisha, for then it is b'Yado (he could make it Tamei) at the time he tells him.

(Rava): Reuven is not believed only when he told Shimon the first time he saw him (after, according to Reuven, it became Tamei).

Yevamos 87b (Mishnah): If one witness said 'You ate Chelev' and the person denied it, he is exempt.

Question: This implies that had he been silent, the witness would have been believed to obligate a Korban, mid'Oraisa. What is the source for this?

Answer #1 (Beraisa): "Or if his sin became known to him (he brings a Korban)", but not if witnesses told him (and he denies it). If he does not deny it he is obligated - "or his sin became known to him" applies no matter how he found out.

If two witnesses came and he did not contradict them, no verse is need to obligate him! Rather, one witness came and he did not contradict him.

Objection: Perhaps the witness is not believed. Rather, silence is like admission!

Support (Seifa): If two witnesses say 'you ate Chelev', and he says 'I did not eat', he is exempt;

R. Meir says, he must bring a Korban.

In the Reisha (with one witness), Chachamim do not obligate because the witness is believed. Chachamim exempt even when there are two witnesses and he contradicts them! Rather, his silence is like an admission.

Rishonim

The Rif (Kidushin 28b) brings Abaye's teachings.

Rosh (Gitin 5:8:13): If the owner was silent when one witness said that his item is Asur, this is like admission (Kidushin 66a). Yevamos 87b says that silence is like admission. This is only if the witness says that the owner saw it or saw Raglayim l'Davar, similar to the case of 'you ate Chelev'. If not, he is silent because he does not know, and one witness cannot forbid if it was not b'Yado.

Question: In Kidushin Abaye and Rava agree that one witness is believed if he told Levi 'your Taharos became Tamei' or 'you ate Chelev' and Levi was quiet. In Gitin (54b), Abaye and Rava believe a witness only if the matter was b'Yado!

Answer (Rosh, citing R. Tam): In Gitin, Levi contradicts the witness or says 'I do not know'. In Kidushin, Levi is silent. Yevamos (88a) proves that this is so. The Gemara could not prove that one witness is believed, for perhaps silence is like admission. If Reuven says that Levi's wine was poured to idolatry and Levi was silent, Reuven is believed even if he did not tell him the first time he saw him and it was not b'Yado then. This is if Reuven says that Levi saw it or saw Raglayim l'Davar (supporting evidence), similar to the case of 'you ate Chelev'. If not, Levi is silent because he does not know. The Ritzva says that even when he should have known and was initially silent and later protested, if he says that his initial silence was not an admission, rather he was thinking whether or not it is true, it is not an admission. This is like Bava Metzi'a 37a (if different people told Shimon 'you stole from me', Shimon can say that he was silent because he was thinking whether or not this is the one from whom he stole).

Beis Yosef (YD 127 DH ha'Omer): The laws of R. Tam and the Ritzva are both true. The Mordechai says that when one must contradict the witness, the contradiction must be Toch Kedai Dibur. Saying 'perhaps you were not precise' is considered contradiction.

Nimukei Yosef (27b DH mid'Oraisa): In Kidushin, Abaye teaches that one witness is believed, i.e. we rely on the witness, not that silence is like admission. In some of his cases the owner would not know whether or not it is true. This is like the version in Kerisus that Chachamim exempt because one is believed about himself more than 100 witnesses. Our Gemara concludes like the other version in Kerisus, that Chachamim exempt because we interpret his words to say that he ate b'Mezid, not b'Shogeg; witnesses do not know this. If Chachamim agreed that there is testimony about Shogeg, and this is why they agree that one who is silent to one witness is liable, they would not exempt when there are two witnesses even if he contradicts them! It is difficult to say that also according to the other version Chachamim hold that there is no testimony about Shogeg, and they address R. Meir according to his opinion (and say that in any case one is believed about himself more than witnesses). We hold like Abaye.

Poskim

Shulchan Aruch (YD 127:1): If Reuven told Shimon 'your wine was poured to idolatry' and Reuven had no opportunity to do this himself and he did not tell Shimon at the first opportunity, he is not believed if Shimon contradicts him (Toch Kedai Dibur) or says that he does not know. If Shimon was silent, this is like an admission.

Beis Yosef (DH veha'Rashba): The Rashba says that a worker is normally careful, therefore if he did not tell him at the first opportunity he is not believed later. If the witness was not a worker we do not distinguish.

Question (Darchei Moshe 4): One who is not a worker is believed due to the owner's admission. Clearly, it does not matter whether or not he told him at the first opportunity!

Answer #1 (Bach DH u'Mah she'Chosav u'Mah): One might have thought that if he did not tell him at the first opportunity he surely intends merely to vex him, and the owner did not bother to answer him.

Answer #2 (Shach 9): The Rashba (Teshuvah 376) says that Abaye believes the witness. He does not rely on the owner's silence; even if the owner is unsure, it is forbidden. Tosfos, the Rosh and the Tur rely on his admission.

Shach (10): Tosfos says that silence is not a total admission. If it were, we would not need the witness' credibility at all! We find that if the witness was a thief, silence is not admission (Kidushin 66a)! Rather, silence shows that he recognizes Raglayim l'Davar and relies on the witness.

Rema: If Shimon did not contradict the owner it is forbidden even if another witness (David) contradicts Reuven, since Shimon is silent.

Rebuttal (Shach 12): When David contradicts Reuven, Shimon is silent because he relies on David! Perhaps Even if we forbid l'Chatchilah, surely b'Di'eved it is permitted to David.

Shulchan Aruch (ibid.): This is if he says 'it was poured to idolatry in front of you', or you know Raglayim l'Davar. If Shimon was silent and later contradicted him and said that his silence was not an admission, rather, he was trying to remember whether or not it is true, Shimon is believed.

Shach (16) and Gra (12): This is like Tosfos and the Rosh. The Rashba and many others rely on the witness (not on the admission).

Shach (17): If later Shimon does not contradict him, just he says that he is not sure, all agree that the witness is believed.

Rema: If Shimon and Levi were partners, and Reuven told them 'your wine was poured to idolatry' and Shimon was silent and Levi contradicted him, the wine is forbidden to Shimon and permitted to Levi.