Kesubos, Chapter 2, Mishnah 2. This mishnah relates back to the opinion of Rabbi Yehoshua taught in the first chapter, in Mishnah 6, where Rabban Gamliel and Rabbi Yehoshua disagreed about a case in which the woman was found not to be a virgin, and she claims "mishe'erastani nenesati" - after you betrothed me, I was raped.
The dispute in the first chapter:
The woman's claim: After I was already betrothed to you and the kiddushin had taken place, I was raped. That is why I am not a virgin, and it is not my fault. The incident happened while I was under your authority, so the loss is yours, and you married me as a virgin - therefore I am entitled to a kesubah of two hundred.
The husband's claim: Not so. You entered the marriage already not a virgin, and I was deceived. This is a mistaken transaction, and I owe you no kesubah at all.
In that mishnah, Rabban Gamliel holds that she is believed, while Rabbi Yehoshua holds that she is not believed.
The migo within her claim:
A point that was not mentioned when studying that mishnah is that even according to Rabbi Yehoshua it would seem that the woman has what is called a 'migo': she had a better claim available to her, and the fact that she did not use it should - or at least one can argue that it should - lend her credibility.
What is that claim? She could have said that she never had relations with any man, and the reason she was found not to be a virgin is that she is mukas eitz - she was merely injured. She did not make that claim; instead she claimed that she did have relations, but that they were forced upon her after the betrothal. Nevertheless, despite the migo she has, Rabbi Yehoshua rules that she is not believed.
Our mishnah: And Rabbi Yehoshua concedes:
Our mishnah presents a case in which Rabbi Yehoshua himself makes use of a claim of this type, except that here the migo is larger and stronger, and the case is entirely different, having no connection at all to the laws of marriage:
"U'modeh Rabbi Yehoshua b'omer la'chaveiro: sadeh zu shel avicha haysah u'lekachtiha heimenu - shehu ne'eman, shehapeh she'asar hu hapeh shehitir".
A person who says to his fellow: the field I hold belonged to your father, and I purchased it from him - is believed even according to Rabbi Yehoshua. The reason: "shehapeh she'asar hu hapeh shehitir" - the mouth that forbade is the mouth that permitted. The mouth that forbade is the statement that took the field out of his possession, for until now it was not known at all that the field had belonged to the father, and he is the one who revealed it. But that very same mouth is what permitted, for in the same breath he adds that he bought it from him. In other words, the one who presented the problem also presented its solution: our knowledge of the father's ownership comes from him alone, and at that very moment he says that he purchased the field from him - therefore he is believed.
The difference from the case in the first chapter:
This is different from the case in the first chapter, where the woman was found not to be a virgin. The new information did not come from her mouth but was established against her will, and the question is how to explain it. In such a case there is no claim of 'the mouth that forbade is the mouth that permitted', since there is no person here who introduced the information and also provided its explanation.
When there are witnesses:
In contrast, the mishnah continues and rules that if there are witnesses who testify that the field belonged to his father, and he says "lekachtiha heimenu" - I purchased it from him - he is not believed. The reason: there is no mouth here that forbade. It was not from him that we learned that the field belonged to the father, but from outside information not from his mouth. True, he claims that he bought it from the father, but that claim alone is not enough to believe him and to establish that the field passed into his possession.
In summary: In this mishnah we learned that even Rabbi Yehoshua, who does not believe the woman who claims "mishe'erastani nenesati" on the strength of the migo she has, concedes that in a case of 'the mouth that forbade is the mouth that permitted' the claimant is believed. The difference lies in the source of the knowledge: when the person himself introduced the information and in the same breath also explained it - he is believed; but when the information comes from outside, through witnesses or through the established facts, his explanation alone is no longer enough to win his case.