Sukkah 43A

ויעבירנו ארבע אמות ברשות הרבים. והיינו טעמא דשופר, והיינו טעמא דמגילה.

and in doing so carry it 4 cubits in the public domain, thereby violating a severe Torah prohibition. And that is the reason for the prohibition against sounding the shofar on Shabbat, and that is the reason for the prohibition against reading the Scroll of Esther when Purim coincides with Shabbat.

אי הכי יום ראשון נמי! ראשון, הא תקינו ליה רבנן בביתו. התינח אחר תקנה, קודם תקנה מאי איכא למימר!

The Talmud asks: If so, on the 1st day of Sukkot that coincides with Shabbat too one should not take the lulav due to this concern. The Talmud answers: With regard to the 1st day, the rabbis instituted that one should take the 4 species in his house. Since the rabbis already prohibited one from taking the lulav out of the house, he will remember that it is prohibited and will not come to take it elsewhere to learn to wave it or to recite the blessing. The Talmud asks: This works out well after the ordinance that one takes the lulav in his house was instituted. However, prior to introducing the ordinance, what is there to say in explaining why it is permitted to take the lulav on the 1st day?

אלא, ראשון דאיתיה מן התורה בגבולין ⁠— לא גזרו בהו רבנן. הנך דליתנהו מן התורה בגבולין ⁠— גזרו בהו רבנן.

Rather, the Talmud rejects the previous explanation and explains the distinction differently. On the 1st day, when the mitzva of taking the lulav even in the outlying areas and not just in the Temple is in effect by Torah law, the rabbis did not issue a decree to prohibit taking the lulav on the 1st day and permitted the mitzva to be performed even in the outlying areas. However, with regard to these other days of Sukkot, when the mitzva of taking the lulav is not in effect by Torah law in the outlying areas and the lulav is taken there only to commemorate the practice in the Temple, the rabbis issued a decree to prohibit taking the lulav on the other days.