Eruvin 103A

אי אליבא דרבי אליעזר קאמר, אפילו לכתחילה נמי.

If he stated his ruling in accordance with the opinion of R' Eliezer, why did he permit tying a bow only after the fact? It should also be permitted even ab initio, as R' Eliezer maintains that preparations required for the performance of a mitzva override the prohibitions of Shabbat.

אלא, לא קשיא: הא ⁠— רבי שמעון, הא ⁠— רבנן, דתניא: בן לוי שנפסקה לו נימא בכנור ⁠— קושרה, רבי שמעון אומר: עונבה.

Rather, the Talmud rejects the previous explanation: It is not difficult; this baraita that deems tying prohibited is according to the opinion of R' Shimon, while that Mishnah that rules that tying is permitted, is according to the opinion of the Rabbis. As it was taught in a baraita: If a string of the Levite’s harp was severed on Shabbat, he may tie it with a knot; R' Shimon says: He may only form a bow. The Rabbis permit the preparations for a mitzva that could not have been performed before Shabbat, whereas R' Shimon is stringent and prohibits even those preparations.

רבי שמעון בן אלעזר אומר: אף היא אינה משמעת את הקול. אלא משלשל מלמטה וכורך מלמעלה, או משלשל מלמעלה וכורך מלמטה.

The Talmud continues its citation of the baraita. R' Shimon ben Elazar says: Even if he ties a knot or a bow, the harp will not issue the proper sound, and he would therefore be committing a transgression without performing the mitzva in a fitting manner. Rather, he unwinds the string from the lower knob and winds it around the upper one, or he unwinds the string from the upper knob and winds it around the lower one, before tightening the string until it produces the proper note.

ואיבעית אימא: הא והא רבנן, ולא קשיא: כאן באמצע, כאן מן הצד.

And if you wish, say instead that both sources were taught in accordance with the opinion of the Rabbis, who permit preparations for a mitzva that could not have been performed the day before, and even so it is not difficult; here, the Mishnah permits tying in a case where the string was severed in the middle, in which case the sound would be affected if the string were reconnected with a bow, whereas there, the baraita is referring to a string that was severed on the side near the end of the string, which can be fixed with a bow.