Gemara

113b (Rav Yehudah): If three men came to visit a sick man (and he commanded who should inherit his property), they may write down his command, or execute it, whichever they want. If two came, they write down his command, but they may not execute it.

(Rav Chisda): This applies only during the day. At night, even three can only write, and they cannot execute (even the next day).

This is because when they heard the command they were able to be only witnesses, not judges, and a witness cannot be a judge.

Sanhedrin 28a - Question: What is the source that two relatives cannot testify together (about someone else)?

Answer #1 (Rami bar Chama): Reasoning teaches this. Witnesses are not made Zomemim unless both are Huzmu. If relatives could testify about someone else, they could be killed due to each other's testimony!

Objection (Rava - Mishnah): If David testified about three years of Chazakah, and three brothers testified, each about one of the three years, (this is testimony of Chazakah, for) testimony of each year is separate. It is like one testimony for Hazamah (they are exempt unless all of them are Huzmu).

Question: If they will be Huzmu, the brothers will pay due to each other's testimony!

Answer: You must say that they pay due to the Hazamah, not due to each other's testimony. Likewise, if relatives testified together and were Huzmu, they are killed due to the Hazamah, not due to each other's testimony!

Answer #2 (Rava): Rather, the Torah could have said 'Ben Al Avos' or 'Hem Al Avos.' Instead, it says "u'Vanim" to teach that Banim (i.e. relatives) cannot testify with each other.

34a (Beraisa - R. Meir): "V'Al Pihem Yihyeh Kol Riv v'Chol Naga" equates Dinim and Tzara'as. Just like the Kohen looks at Tzara'as only during the day ("uv'Yom Hera'os") also Dinim must be judged during the day.

R. Meir expounds "b'Yom Hanchilo" like Rabah bar Chinena, to teach that (judgment about) bequeathing is during the day, but not at night.

Rosh Hashanah 25b (Mishnah): If only Beis Din saw the new moon, two of them stand and testify in front of them, and the others are Mekadesh the new month. If three saw the new moon and they are Beis Din, two of them stand and seat from their colleagues with the third, and testify in front of them, and they (those sitting) are Mekadesh the new month.

Question: Why must two stand and testify? Hearing (testimony) is no better than seeing! (The judges themselves saw the new moon!)

Answer (R. Zeira): The case is, they saw at night.

Suggestion: In the Seifa, a witness becomes a judge. This is unlike R. Akiva!

(Beraisa - Rebbi Tarfon): If the Sanhedrin saw a murder, some members testify on this in front of other members (who judge the case);

R. Akiva says, they are all witnesses, and a witness cannot become a judge.

Rejection: R. Akiva says so only about capital cases, for it says "the congregation will judge and the congregation will save." Judges who saw the murder will not see a reason to acquit. He could agree about Kidush ha'Chodesh.

Rishonim

The Rif brings the Gemara in Bava Basra.

Ran (Kesuvos 8b DH HIlkach): Some say that only for Kidush ha'Chodesh we say that hearing (testimony) is no better than seeing, for the Torah did not mention Hagadah (verbal testimony).

Rambam (Hilchos Edus 5:8): In capital cases, a witness who testified may not give a reason to convict or acquit the Nidon. If he says that he has a reason to acquit the Nidon, we silence him, for it says "Lo Ya'aneh b'Nefesh Lamus" - to acquit or convict. In monetary cases, a witness may give reasons to convict or acquit the Nidon, but he may not be counted among the judges. A witness cannot become a judge even in monetary cases.

Rambam (Hilchos Sanhedrin 3:6): If two came to visit a sick man and he commanded in front of them, they write down his command, but may not execute it. If three came, they may write down his command, or execute it, whichever they want. If two came, they write down his command, but may not execute it. This applies only during the day. At night, even three can only write, and they cannot execute.

Question: Kidush ha'Chodesh and validation of documents do not need Hagadah, but other monetary cases do!

Answer #1 (Kesef Mishneh, citing the Ritva): Since he cmd in front of them, this is like Hagadah.

Answer #2 (Kesef Mishneh, citing the Nimukei Yosef): Rashi holds that he made them a Beis Din, so they can rule based on what they saw. Since they know the truth, they do not need testimony. Others say that Hagadah is needed; one's admission is like 100 witnesses.

Kesef Mishneh: It seems that Rashi requires Hagadah! He said that they can act like they heard. This connotes that that the command is like Hagadah.

Tosfos (Bava Kama 90b DH Kegon (1)): Some say that a witness cannot become a judge because he would not accept Hazamah about himself. The Ri of Orlins disagrees, for if so, also relatives of the judges should not be able to testify in front of them, for they cannot be Huzam. We do not say so, for they can be Huzam in another Beis Din. For the same reason, a witness can become a judge! The Ri says that we learn from "b'Yom Hanchilo" that judgment about bequeathing is not at night, i.e. if three heard a command at night they cannot execute it, because they were witnesses, and cannot become judges.

Poskim

Shulchan Aruch (CM 7:5): A witness who testified cannot become a judge (about this case). If one of the judges testified in front of his collegues about what he saw, he cannot join with them to judge it.

Beis Yosef (DH Ein): The Rashbam learns from "the two men (witnesses) will stand in front of Hash-m (the judges)." Others say that it is because it is Iy Efshar Lehazimah. This is only if he testified. If he need not testify, e.g. there are other witnesses, or Beis Din saw during the day, witnesses can become judges. This refers to mid'Oraisa matters. For mid'Rabanan matters, even a witness who testified can become a judge.

SMA (14): He cannot judge only if he testified in front of three colleagues. Ir Shushan disqualifies if he testified in front of one colleague. This is wrong.

Shach (4): Intent to testify makes one a witness regarding if one of the witnesses was found to be a relative or Pasul. Even though the Ran equates this to witnesses becoming judges, we follow Tosfos and the Rosh, who distinguish. Witnesses cannot become judges due to "the two men will stand in front of Hash-m", or because it is Iy Efshar Lehazimah. These apply only to a witness who testified.

Shulchan Aruch (ibid): If he did not testify, e.g. three judges saw an act, even if they intended to be witnesses, if they saw during the day they become judges.

SMA (13): Witnesses cannot become judges because the testimony cannot be Huzam. This does not apply when three judges saw together.

Shulchan Aruch (ibid): If they saw at night, they cannot judge by themselves; others must testify in front of them.

Bach (DH v'Tu): Why did the Tur write 'at a time when we can judge', and not 'during the day', like Tosfos and the other Poskim? It seems that he includes Shabbos and Yom Tov. Even though the Isur to judge then is mid'Rabanan, Chachamim enacted like mid'Oraisa, and they may not judge based on what they saw then.

Rebuttal (Taz DH Im): In Siman 253, the Tur himself says 'during the day'! Also, what is the source for this Chidush? The Poskim discuss only days when the Torah forbids judging. They did not teach this bigger Chidush! Also, witnesses can become judges for mid'Rabanan testimony. Here, we do not say that Chachamim enacted like mid'Oraisa!

Rema: This refers to mid'Oraisa matters. For mid'Rabanan matters, witnesses can become judges.