Gemara

Yevamos 31b: We didn't enact a date on a document of Kidushin, since it would not (always) enable us to know when she is Chayav Misah:

If we would enact that she keeps the document, she would erase the date (we cannot kill her unless we are sure)!

If we would enact that he keeps it, if he has compassion on her, e.g. when she is his sister's daughter, he will (erase the date to) cover up for her!

Suggestion: We should enact to leave it by the witnesses.

Rejection: If they remember the date, they do not need the document;

If they do not remember the date, perhaps they will testify based on what they see written. Testimony must be mi'Pihem ("Al Pi Shnei Edim...", from their mouths), not mi'Pi Kesavam (from what they wrote)!

Gitin 71a (Rav Kahana): If a Cheresh (deaf person) can communicate by writing, we write and give a Get to his wife if he requests this.

Question (R. Zeira - Beraisa): "If he will not testify" excludes a mute, for he cannot testify.

Why can't he testify by writing?

Answer (Abaye): Testimony must be mi'Pihem, not mi'Pi Kesavam.

Question (Beraisa): Just like we test him (one who became mute, to see if his mind is sound enough) to give a Get, we also test for sales, gifts, testimony, and inheritances.

This shows that he can testify, even though he cannot speak!

Answer (Rav Yosef bar Minyomi): The Beraisa refers to Edus Ishah (testimony that a man died to permit his wife to remarry). Chachamim were lenient about Edus Ishah.

Kesuvos 20a (Beraisa): A person may write in a document testimony (that he saw), and (orally) testify from it many years later.

Opinion #1 (Rav Huna): He can testify only if he remembers the testimony by himself.

Opinion #2 (R. Yochanan): He can testify even if he does not remember it by himself.

(Rabah): We learn from R. Yochanan that if two witnesses saw testimony and one forgot it, the other may remind him.

Rishonim

Rif and Rosh (Kesuvos 7b and 2:16): Rav Hai Gaon says that the Heter to write testimony in a document refers to a witness who saw testimony, and much later the lender asked him to write a document or testify in Beis Din. This is permitted, even after many years. He need not be concerned that the loan was paid or pardoned. Rav Huna requires that the witness remember the testimony by himself; R. Yochanan allows even if he remembers after others remind him. Rava learns from R. Yochanan that if two witnesses saw testimony and one forgot it, the other may remind him. Some say that the Gemara does not discuss a document given to the lender, rather, a Sefer of recollections one keeps in his house. A party asked him to testify, and he finds in his Sefer the details of the testimony. He may testify even after many years. Rav Huna requires the witness to remember the testimony by himself; R. Yochanan allows even if he remembers after seeing it written. According to this, Rava derives a Chidush from R. Yochanan that if one witness forgot the testimony, the other may remind him. According to the first Perush, this is (part of) R. Yochanan's Chidush; what does Rava add?! Therefore, the latter Perush is better.

Ran (DH Kosev): Also according to the first Perush, Rava adds to R. Yochanan's Chidush! One might have thought that others can remind a witness (Reuven), for he would not rely on them if he did not remember, but we do not allow the other witness to remind Reuven, lest Reuven rely on him even though Reuven himself does not remember. However, the Tosefta favors the Rif's Perush.

Rosh: It seems that in any case one may rely on Rav Hai Gaon's Perush to write testimony even many years later without concern that the loan was paid or pardoned. Had the borrower paid it, he would have told the witnesses so they will not write or testify about the loan. If he did not do so, he caused himself a loss.

Nimukei Yosef (Yevamos 9a DH Linsei): "Al Pi Shnei Edim" teaches that witnesses may not testify from what they wrote (unless they remember it), and also that one may not write his testimony and send it to Beis Din.

Rambam (Hilchos Edus 3:4): The Torah requires testimony to be from the mouth of witnesses, "Al Pi Shnei Edim...", not mi'Pi Kesavam. Mid'Rabanan we decide monetary cases based on documents, even if the witnesses are not alive, lest people be discouraged from lending. We do not rule about fines based on documents, and all the more so not about lashes or Galus. Those must be mi'Pihem, not mi'Pi Kesavam.

Rambam (8:5): Whether one writes testimony in a document, or it is found in his ledger in his handwriting 'Ploni made me a witness on this day about this matter...', he may testify only it he remembers the testimony by himself or when reminded of it. If not, this is Ed mi'Pi Ed. It is like one (David) who heard from a reliable source that Reuven owes Shimon a certain amount, and David testified even though he does not know.

Rebuttal (Ra'avad): The first law is wrong. (Rather, one may testify about what he wrote in a document even if he does not remember it.)

Radvaz and Migdal Oz: The Ra'avad rules like R. Yochanan because Rava derives another law from R. Yochanan.

Kesef Mishneh: The Gemara permits to testify based on one's own written testimony only if he remembers it. The Rambam explains that this is whether he wrote in his ledger or in a document. The Gemara supports this; it says that one may write in a document. The case is, the document cannot be validated without the witness. The same applies to testimony in one's ledger.

Ra'avad: Such testimony can be validated. E.g. if the witnesses had the document and the borrower said 'do not give it to the lender until he gives to me the money', and one witness is unsure whether or not the borrower admitted that he gave. He remembers after seeing the document or being reminded by the other witness.

Rambam (9:11): Even a Cheresh that can speak but not hear, or vice-versa cannot testify, even if he sees fine and has a sound mind. Testimony in Beis Din must be oral, or by one who could testify orally and hear the judges' warning. The exception is Edus Ishah, for which Chachamim were lenient.

Question (Rosh Yevamos 3:7): We learned (Bava Basra 168a) that if a document faded, the bearer brings witnesses to Beis Din, and they write a Kiyum. If a document was burned and witnesses know what it said, they can write another document (Bava Kama 98b)! These show that when witnesses say what they saw written in a document, we fulfill mi'Pihem v'Lo mi'Pi Kesavam, all the more so when the witnesses who signed say that they signed it!

Answer (Rosh): When witnesses say that they saw a signed document and recognized the signatures, this is not mi'Pi Kesavam. It is like normal Kiyum of a document. It is as if their testimony was investigated in Beis Din. Also here (if we would leave a Shtar Kidushin with witnesses), if they would testify that they signed it, this is mi'Pihem. The concern is lest they testify about what happened as if they remember, but really they do not; they only see what they signed. This is mi'Pi Kesavam. When witnesses were far from Beis Din, R. Tam used to have them write their testimony and send it to Beis Din. This is mi'Pihem because they remember it. However, Rashi (Devarim 19:15) says that "Al Pi Shnei Edim" disallows this. We learned that "If he will not testify" excludes a mute, for he cannot testify. He cannot testify by writing because testimony must be mi'Pihem, not mi'Pi Kesavam (Gitin 71a). This is not a proof for Rashi. Perhaps a mute is different, for he is not able to testify orally. What I wrote is like Tosfos; it is better than what the Rif wrote.

Question (Beis Yosef CM 28 DH Kosav Rashi): If a person wrote testimony that he saw, he may testify about it later only if he remembers the testimony (Kesuvos 20a). According to R. Tam, even if he does not remember, Beis Din may rely on what he wrote!

Answer (Beis Yosef): The case is, the witness does not have in Beis Din what he wrote.

Ritva (Makos 6b DH she'Lo): Rashi agrees that a witness in Beis Din may write his testimony, give it to the judges and say 'this is my testimony'. This is mi'Pihem. He disallows only sending written testimony to Beis Din.

Poskim

Shulchan Aruch (28:12): We do not rule about fines based on documents, and all the more so not about lashes or Galus. Those must be mi'Pihem v'Lo mi'Pi Kesavam.

SMA (25): R. Tam says that witnesses can write their testimony and send it to Beis Din. This is only if the witnesses can speak, and could fulfill "Al Pi Shnei Edim..."

Aruch ha'Shulchan (16): If written testimony was never valid, we would not need a Mi'ut to disqualify a mute from testimony! Most Poskim are unlike R. Tam. However, some Poskim rely on R. Tam in pressed circumstances to accept written testimony from a Chacham. All allow a witness in Beis Din to write his testimony and give it to the judges. The judges must show it to the parties, for Kabalas Edus must be in front of the parties.

Shulchan Aruch (ibid.): As long as one remembers he may testify. He need not be concerned that since it has been a long time he does not remember well.

Shulchan Aruch (13): Even if a witness forgot testimony and remembers it only after seeing what he wrote in his ledger, and then he remembers the testimony, he may testify.

Beis Yosef (DH u'Mah she'Chosav v'Afilu): Even though most disagree with Rav Hai Gaon's Perush, the law is true.