Ketubot, Chapter Two, Mishnah Two
At the end of the last chapter there was a series of debates in which Rabbi Joshua consistently did not believe the woman and Rabban Gamaliel and Rabbi Eliezer did. Our mishnah contains a case where Rabbi Joshua does believe the claim made (this time by a man). The reason why he believes the man in this case is that he invokes a principle called, the mouth that forbade is the mouth that permitted. This halakhic principle means that if a person says something which makes something forbidden to him he is also believed when he says something to make that very same thing permitted to him. The next few mishnayoth will illustrate this principle and limit its applicability.
1) And Rabbi Joshua admits that, if one says to his fellow, This field belonged to your father and I bought it from him, he is believed, for the mouth that forbade is the mouth that permitted.
2) But if there are witnesses that it belonged to his father and he says, I bought it from him, he is not believed.
In this case Reuven approaches Shimon and tells him that the field that is currently in Reuvens possession was purchased from Shimons father. Shimon did not approach Reuven first claiming the field, nor is there any other evidence that the field once belonged to Shimons father. Indeed, without Reuven having told Shimon that the field once belonged to Shimons father, we would have thought that the field was always Reuvens. In this case Reuven is the mouth that forbade when he said that the field once belonged to Shimons father. He made a statement that was detrimental to himself. Since he is the mouth that forbade, he is believed to be the mouth that permits and state that he purchased the field from Shimons father. Reuven is believed even if he produces no evidence that he bought the field. Had Reuven kept his mouth shut, Shimon would never have known that the field once belonged to his father. Therefore, Reuven is believed when he says that it used to belong to Shimons father but he bought it from him.
Section two: In contrast, if witnesses come and state that the field was once Shimons fathers field, then Reuven is not the mouth that forbade. He is only the mouth that permits, and he is therefore not believed. After all, had he kept his mouth shut, the field would have been taken over by Shimon. In order to retain possession of the field he will need to bring proof that he bought it.