Is one required to stand during the Chazan's repetition of the Amida?
The Rama (Rabbi Moshe Isserles, Poland, 1525-1572), in his glosses to the Shulchan Aruch (124:4), cites the position of the Sefer Ha'minhagim requiring that one stand during the repetition of the Amida. This is the view as well of Rabbi Yitzchak Abuhav (Spain-Israel, 1433-1493), in his work Menorat Ha'ma'or (3:111; listen to audio for precise citation).
Some writers claimed that the Rambam (Rabbi Moshe Ben Maimon, Spain-North Africa, 1135-1204) held this opinion, too, as he writes (Hilchot Tefila 9:3), "Everyone stands and listens to the repetition of the Shali'ach Tzibur." However, Chacham Ovadia Hadaya (Israel, 1890-1969), in his work Yaskil Avdi (vol. 2, Orach Chayim, 2), refutes this claim, arguing that the word "Omedim" (literally, "standing") can also be used in reference to silence, in which case the Rambam here speaks not of standing during the repetition, but rather of remaining silent. Furthermore, had the Rambam held that one must stand during the repetition, the Rama would have cited the Rambam as the source for his ruling, rather than citing the Sefer Ha'minhagim.
Conversely, Rabbi Yaakov Chagiz (1620-1674), in his work Halchot Ketanot, suggests proving that one may sit during the repetition of the Amida from the Gemara's comment (in Masechet Yoma 87b) that Shemuel would rise for the recitation of Viduy (confessional) during the Yom Kippur prayer service. If Shemuel had to rise for the Viduy, then he must have been sitting during the repetition of the Amida, thus proving that one need not stand during the repetition. Others, however, refuted this proof, arguing that Shemuel perhaps sat specifically on Yom Kippur because he felt weak and frail as a result of fasting.
As for the final Halacha, Chacham David Yosef, in his work Halacha Berura (vol. 6, p. 271), writes that those who have the practice to stand for the repetition should continue this practice, as it is rooted in several Halachic sources. He adds, however, that somebody who feels ill or frail may be lenient and sit during the repetition even if he normally follows the practice to stand. Those who follow the practice to sit during the repetition have authorities on whom to rely and may continue doing so, and this is, indeed, the custom among most Sepharadim. However, a person who prays in a congregation where the practice is to stand during the Amida should abide by the local custom and stand, even if his personal practice is to sit. In the converse situation, one may stand during the repetition in a congregation where the custom is to sit.
