One who sits may not testify.

A sitting Chacham may testify.

Shevuos 30b: Witnesses must stand. It says "v'Omdu Shnei ha'Anashim."

Rav Yeimar knew testimony for Mar Zutra. Ameimar told everyone to sit.

Question (Rav Ashi): Ula taught that R. Yehudah and Chachamim agree that the witnesses must stand!

Answer (Ameimar): It is a Mitzvas Aseh for the witnesses to stand, and an Aseh to honor Chachamim. The Aseh to honor Chachamim takes precedence.

Gitin 71a (Beraisa): We test one who became mute (to see if he is alert enough to give a Get), and similarly regarding sales and gifts, for testimony, and for inheritances.

Rambam (Hilchos Sanhedrin 21:5): The custom of all Batei Din after the Gemara, in all Yeshivos, is to seat the litigants and witnesses to avoid arguments. We are unable to practice all the laws properly.

Hagahos Maimoniyos (2): One must be careful not to rule about a Get through testimony given sitting, for the Gemara says that sitting is Pasul for testimony. B'Di'eved it is Kosher, like the Yerushalmi says about judges who accepted testimony standing. Both of these are learned from the same verse.

Rosh (Shevuos 4:5): Some say that if witnesses testified sitting, b'Di'eved the testimony is invalid, for it says in Zevachim that sitting is Pasul for testimony. Pasul connotes b'Di'eved. Also, if judges stood while accepting the testimony, it is Pasul. This is wrong. The Yerushalmi says that the latter is Kosher.

Ran (Shevuos 14a DH Garsinan): Some say that if witnesses testified sitting, b'Di'eved the testimony is invalid, for it says in Zevachim that sitting is Pasul for testimony. Pasul connotes b'Di'eved. This is wrong. The episode with Rav Yeimar proves that a Chacham testifies sitting, for the Aseh to honor Chachamim takes precedence. If even b'Di'eved the testimony is Batel, how can we seat a Chacham?! Are we Machshir Pasul testimony due to honor of Torah?! We should rather tell a Chacham not to testify at all, like we say about a king (Sanhedrin 18a)! Rather, b'Di'eved it is Kosher. The Rashba proved this from Gitin 71a, in which we say that we check one who became mute for sales, gifts, testimony, and inheritances. In the Yerushalmi, Reish Lakish learned from here that one may testify sitting. This shows that b'Di'eved it is Kosher.

Rashba (6:208): Witnesses must stand, i.e. l'Chatchilah. B'Di'eved, the testimony is valid even if they sat. The Yerushalmi says so. A Chacham testifies sitting l'Chatchilah.We say that sitting is Pasul for testimony l'Chatchilah. The Ge'onim agreed to this.

Shiltei ha'Giborim (Sanhedrin 1b-2a): We can be lenient to let witnesses sit only for monetary cases. Regarding Isur, e.g. witnesses testify to permit or forbid a woman to her husband, Chachamim do not have authority to be Machshir testimony given sitting. The witnesses could retract from it. Similar, witnesses who testified at night can retract, since investigating witnesses is like the beginning of judgment, which must be during the day.

Shulchan Aruch (CM 17:1): L'Chatchilah, the witnesses always stand.

Beis Yosef (DH u'Mah she'Chosav v'Davka): It seems that also the Rambam holds that if the witnesses testified sitting, it is Kosher. We would not be Machshir Pasul testimony to avoid arguments! R. Elazar b'R. Shimon says that the Chiyuv to stand is in front of expert judges - "v'Omdu Shnei ha'Anashim Lifnei Hash-m." Nowadays we do not have expert judges, so the custom is to sit.

SMA (5 and Hagahos Drishah u'Prishah 2): The verse "v'Omdu..." is only an Asmachta, unlike Ir Shushan, who says that it was ppeven b'Di'eved when there were expert judges. The Mordechai said that R. Elazar b'R. Shimon permits l'Chatchilah nowadays, but he did not say that it was Me'akev when there were experts. Also, R. Elazar b'R. Shimon is a lone opinion. No Posek brings his opinion. The Rambam and Tur say that it is an enactment to let witnesses sit.

Bach (5): Even though early Chachamim were concerned and permitted them to stand to evade arguments, if the witnesses and litigants fear Hash-m, they will be concerned for the honor of Torah and Mitzvos, and stand, to fulfill "v'Omdu Shnei ha'Anashim Lifnei Hash-m." Even though the judges tell them to sit, they should reply 'we stand in front of Hash-m and in front of you, for judgment is to Hash-m.' What is the source that the verse is only l'Chatchilah? Perhaps it is even b'Di'eved, but only when there are expert judges. Chachamim enacted to stand even nowadays, but permitted sitting to avoid arguments. They did not enactwere it may lead to problems. It seems that all the Poskim hold like R. Elazar b'R. Shimon, unlike SMA says.

Shach (28:11): The Mabit (2:116) says that if judges accepted testimony standing in a place where they do not normally judge, e.g. in the market or on the road, even b'Di'eved it is invalid. There is no necessity to say so. Ben Lev brings in the name of the Rivash that it is Kosher b'Di'eved, and said that he did not see anyone who disagrees.

Shulchan Aruch (3): The custom of all Batei Din in Yisrael is to seat the litigants and witnesses to avoid arguments. We are unable to practice all the laws properly.

Shulchan Aruch (28:5): L'Chatchilah, the witnesses must testify standing. If the witness is a Chacham, we seat him.

Beis Yosef (DH uv'Mordechai): An old edition of the Mordechai says that the Mitzvah for litigants to stand is if they were already standing. If they were already sitting, they need not stand. (It says "v'Amdu (they will stand)." It does not say va'Ya'amod (he will stand up)'! R. Yerucham says that regarding a Get, one must be careful that they stand. Hagahos Maimoniyos says that this is only l'Chatchilah.

Shulchan Aruch (OC 268:7): On Shabbos night, after Shemoneh Esre we say va'Ychulu aloud, and standing.

Mishnah Berurah (19): We stand because we testify that Hash-m created the world, and witnesses must stand. It is good to say it with the Tzibur; the entire Tzibur must testify for Hash-m. At least it should be said with two. Some say that an individual who prays does not say it, for one witness does not testify. Some say that he may say it, but he need not stand. It is good that he say it, without intent for testimony, rather, like one who reads in the Torah. If one forgot to say it in the Beis ha'Keneses, he is Yotzei through saying it standing in Kidush.

Shulchan Aruch (271:10): On Shabbos night, in Kidush we say va'Ychulu standing.

Mishnah Berurah (271:45): This is because it is testimony that Hash-m created the world. Even though he already said it, he repeats it to be Motzi his family. The Medrash says that it should be said three times - in Tefilah, after Tefilah and in Kidush.

Shulchan Aruch (422:7): The Mitzvah is to say Hallel standing

Mishnah Berurah (422:28): Hallel is testimony to the miracles that Hash-m did for us. Also, it says "Hallelu Es Hash-m ha'Omdim." However, on Pesach night, since we do not say Hallel all at once, we do not obligate one to stand each time. Also, on Seder night we recline, to show freedom. If one read sitting, b'Di'eved he was Yotzei, even on days when we finish Hallel.

Kaf ha'Chayim (53): Also, since he is at the meal holding his cup, perhaps he will get confused and his cup will fall.