Must the congregation remain standing during the Hazan’s repetition of the Amida prayer, or may they sit?
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Teruma (listen to audio recording for precise citation), writes that one is required to stand throughout the Hazan’s repetition of the Amida. People who are ill, Heaven forbid, or otherwise frail, such as the elderly, who find it difficult to stand, are allowed to sit. Others, however, must remain standing throughout the entire repetition of the Amida. This ruling is based upon the comments of the Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), in his glosses to the Shulhan Aruch (Orah Haim 124:4), where he records the custom to remain standing during the Hazan’s repetition of the Amida.
Hacham Ovadia rules more leniently on this issue, claiming that those who sit during the repetition have authorities on whom to rely. However, this applies only after the completion of Nakdishach and the Beracha of "Ha’Kel Ha’kadosh." According to all views, one must stand during the beginning of the Hazan’s repetition of the Amida, until he completes the Beracha of "Ha’kel Ha’kadosh." Furthermore, Hacham Ovadia writes that one must stand for Modim De’rabbanan, so that he can bow properly, as required by Halacha. Many people remain seated during Modim De’rabbanan, and just move their bodies forward slightly, mistakenly thinking that this suffices as a bow. This practice is incorrect. Those who sit during the Hazan’s repetition of the Amida must ensure to rise for Modim De’rabbanan and bow properly. It should also be noted that even Hacham Ovadia advises that one stand throughout the Hazan’s repetition, writing that "Ha’mahmir Tabo Alav Beracha" – one who is stringent in this regard is deserving of blessing. Even though Hacham Ovadia does not rule as stringently as the Ben Ish Hai, he does maintain that it is preferable to stand.
Interestingly, some authorities inferred from the comments of the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) that he required standing during the repetition of the Amida. In Hilchot Tefila (9:3), the Rambam writes that while the Hazan repeats the Amida, the congregation "stands and listens and recites ‘Amen’ after each and every Beracha" (listen to audio recording for precise citation). At first glance, the Rambam’s description indicates that one is required to stand throughout the Hazan’s repetition of the Amida.
However, Hacham Ovadia Hedaya, in his work Yaskil Abdi, disputes this inference. He notes that the word "Omedim" ("stand") used here by the Rambam often denotes silence, not standing. Indeed, the Gemara in Masechet Sota uses this word in reference to remaining silent. Quite possibly, then, the Rambam requires not standing during the Hazan’s repetition, but rather remaining silent. Thus, there is no proof from this passage that the Rambam maintains that one must stand during the repetition.
Summary: It is preferable to stand during the Hazan’s repetition of the Amida, though those who sit have a basis on which to rely. Certainly, however, one must stand during the beginning of the Amida, until after the Beracha of "Ha’kel Ha’kadosh," and for Modim De’rabbanan, so that he can make a proper bow.
