Gemara

(Beraisa - Rebbi): If Levi says that he has a document and a Chazakah for his land, he must produce the document;

R. Shimon ben Gamliel says, he brings witnesses of Chazakah.

Surey, all agree that the document is enough. R. Shimon ben Gamliel means, he may bring witnesses of Chazakah or the document.

Bava Basra 170a: They argue about whether one must clarify all his claims. Rebbi holds that he must validate the document and bring witnesses of Chazakah. R. Shimon ben Gamliel holds that witnesses of Chazakah suffice.

Rav Yitzchak bar Yosef claimed that R. Aba owed him money. R. Aba said 'I paid you in front of Ploni and Almoni.' R. Yitzchak Nafcha (who was judging the case) said 'let Ploni and Almoni come testify!'

R. Aba: Am I not believed without them? If one borrowed money in front of witnesses (without a document), he need not pay back in front of witnesses!

R. Yitzchak Nafcha: You yourself taught in Rav's name that if one says 'I paid you in front of Ploni and Almoni', they must testify for him!

R. Aba: Rav said that the Halachah follows R. Shimon ben Gamliel. Even Rebbi said to validate the document only in order to clarify his claim!

R. Yitzchak Nafcha: Also I said only that they should testify in order to clarify your claim!

Shevuos 41b: Reuven claimed a Maneh from Shimon. Shimon said 'I paid you in front of Ploni and Almoni.' Ploni and Almoni said 'we never saw this.'

(Rav Sheshes): Shimon is Huchzak Kafran (established to be a liar).

(Rava): No. Anything that is not incumbent on a person, he is not mindful about it.

Rishonim

Rif (Bava Basra 78b): Rebbi holds that one must clarify all his claims: R. Shimon ben Gamliel holds that he need not, since he has a Chazakah. The Halachah follows R. Shimon ben Gamliel. Even Rebbi said only that he must clarify. I.e. when he brings his document, we tell him to clarify (validate) it, even though he also has a Chazakah. If he cannot clarify it, e.g. the witnesses died or went abroad, he relies on the Chazakah. R. Shimon ben Gamliel holds that he need not clarify at all. He relies on the Chazakah l'Chatchilah.

Rosh (Bava Basra 10:27): (The Rosh cites the Rif verbatim, but in his text it says 'the Halachah follows Rebbi.') It seems that this is only if the witnesses died or went away. If they are here, he must clarify. Also R. Chananel (Sanhedrin 23b) says that according to Rebbi, he is not believed if he claims that the document was lost. Also Sefer ha'Mekach (of Rav Hai Gaon) says that if he does not bring the document, his words are Batel. If the document was found to be Batel, also the Chazakah is Batel. Also in Sanhedrin the Gemara connotes that if the witnesses (on the document) were relatives or Pesulim, he did not clarify. The Rashbam (Sof 170a) has a different text (that says 'the Halachah follows R. Shimon ben Gamliel'). The Ge'onim's text is like I wrote.

Rambam (Hilchos To'en 15:1): If Reuven said 'this field is mine' and brought witnesses who know that it was his, and Shimon, who was on the field, brought a document that he bought it from Reuven and witnesses that he ate it the years of Chazakah, we tell Shimon to validate his document. If he validates it, this is good, and he uses the document in the case. If it is impossible to validate it, we rely on witnesses of Chazakah, and Shimon swears Heses that he bought it.

Magid Mishneh: The Rambam holds like the Rif, that if the witnesses died or went abroad, Rebbi admits that we judge based on the Chazakah. The Rif and Rambam imply that if the document was found to be a forgery, the Chazakah is Batel. Therefore, we force him to clarify. This is obvious, for Chazakah works due to the document (it makes us believe that he truly had a document).

Rashba (1:961): A case occurred in which Reuven gave a gift document to Shimon, and Shimon gave it to Levi in Shimon's Kesav Yad (wrote in his own handwriting that he gives it to him). Levi gave it to Leah (his wife) and gave to her Reuven's gift document and Shimon's Kesav Yad. There was a protest against Leah. Shimon's Kesav Yad was not validated. Levi says that no validation is needed, for the witnesses would not sign on the gift if they did not recognize him and know about the gift. If we discuss land, and Leah made a Chazakah in it, she is believed due to the Migo. Even though she has a Migo, she must validate it, like we learned in Bava Basra. The Halachah follows Rebbi. We must investigate if Rebbi requires one to clarify, and if he cannot, the Chazakah is Batel. A Beraisa says that if one ate Peros of a field due to a document, and the document was found to be Batel, the Chazakah is Batel. Or, perhaps l'Chatchilah we exert him to clarify, to make things clear, but if he cannot, he does not lose, like R. Yitzchak b'Rav Yosef said. Shevuos 41b proves that we hold like him. The Halachah follows Rava. R. Tam, the Ramban and the Ba'al ha'Ma'or did not distinguish. They hold that whenever one must clarify, if he did not, he did not lose, even if witnesses deny what he says, like it says in Shevuos, unless he persists in his false claim and contradicts the witnesses, according to the Ramban. Even though I disagree, and it seems that the Rif disagrees, I cannot decide a matter that greater Chachamim than myself argued about.

Poskim

Shulchan Aruch (CM 140:4): If Shimon claimed that he has a document that Reuven sold or gave a gift to him, and he has witnesses that he made a Chazakah for three years, we tell him to validate his document. If he can, this is good. He must pursue to validate it. If it is impossible to validate it, e.g. the witnesses died or went abroad, and no one recognizes their signatures, we rely on witnesses of Chazakah, and Shimon swears Heses that he bought it. If the witnesses are here, his Chazakah does not help. He must bring the witnesses.

SMA (4): This is unlike CM 70:2, in which one who says 'I paid you in front of Ploni and Almoni' must bring them, and if they do not come, he swears. There, one who pays need not pay in front of witnesses, but l'Chatchilah we tell him to clarify his claim so he need not swear. This is why the Mechaber rules that even if Ploni and Almoni contradict him, he swears Heses and he is exempt, for anything that is not incumbent on a person, he is not mindful about it. Here, Chazakah helps only if he says that he had a document and lost it. Since he has a document, he must validate it. The Ran (Shevuos 21b) says so. If captured woman says that she has witnesses that she was not defiled, she need not wait for them to come (before marrying a Kohen). This is a leniency of a captive.

Shulchan Aruch (5): If he claims afterwards that he lost the document, he is not believed. Similarly, if he brought the document and it was found to be Pasul, also the Chazakah is Batel.

SMA (5): This is when the witnesses were relatives or Pesulim, and all the more so if the document was forged.

Bach (3): What should Shimon do if he lost his document?! Should he lose the land, even though he has a Chazakah?! R. Tam says so according to the Rashbam's text. One should not lose if he cannot clarify a claim that he did not need to make. It seems to me that the Tur says 'he is not believed', i.e. to say that he lost the document so he can rely on his Chazakah. Rather, he must swear that he lost it. This is why he says 'and so says Rav Hai Gaon, that if he did not bring the document (the Chazakah is Batel)', i.e. if he has it but does not want to bring it. This proves that it is a forgery. This is like R. Chananel, who says that he must swear that he lost it. If he admits that he has it, he must bring it. If not, his Chazakah is Batel.

Gra (15): The Shulchan Aruch is like R. Chananel and Rav Hai Gaon. Also the Yerushalmi says that Rebbi holds that even if he retracted and said that he lost the document, Rebbi holds that the Chazakah is Batel. The Yerushalmi says that if one ate years of Chazakah due to a document and it was found to be Pasul, Rebbi and R. Shimon ben Gamliel argue about this.