What is Rashi adding in ד״ה לגבל ולתפילה. When he says משום ... does he want to tell that this whole Halacha of 4 mil come from ריש לקיש or is he telling me in general who said what I'm not clear of why he has to say משום ....
חיים יגאל, Queens
The point is that the Halachah that the Gemara states here (that one has to walk 4 Mil in order to find a Minyan for Tefilah, etc.) does not really have any connection to the Sugya. Therefore, Rashi stresses that it is cited only because we have just reported another Halachah that Rebbi Abahu stated in the name of Resh Lakish. We cite this Halachah that Rabbi Abahu stated in name of Resh Lakish as well.
Soמשום means "because." The Gemara mentions this second Halachah about 4 Mil "because" it has already mentioned another Halachah that Rebbi Abahu said in the name of Resh Lakish.
(On a seasonal note, we find something similar in the Hagadah. We say, "This Pesach that our forefathers ate על שום מה" -- meaning, "What is the connection between the Korban Pesach and coming out of Mitzrayim?"
Chag Kasher v'Same'ach,
Dovid Bloom