Sforno on Exodus 17:9/ספורנו על שמות י״ז:ט׳

1ומטה האלוקים בידי, to show the people at what time he would be praying in order that they too would coordinate the time they offered their own prayers with that of Moses. The importance of this harmonising of prayers is demonstrated when the Talmud tells us that in the large synagogue of Alexandria they waved flags so that the people who could not hear the cantor could pray in harmony with him, nonetheless. (Sukkah 51).