Sukkah 31B

בבני כרכין, שהיו מורישין את לולביהן לבני בניהן! אמרו (להם): משם ראיה? אין שעת הדחק ראיה.

involving city dwellers who lived in an area distant from the region where the 4 species grow, who would bequeath their lulavim to their grandchildren, even though they were completely dry. The rabbis said to him: Is there proof from there that species that are dry remain fit for use? Actions taken in exigent circumstances are not proof. In typical circumstances, it would be prohibited to use those species.

קתני מיהת, רבי יהודה אומר: אף יבשין כשרין, מאי לאו, אאתרוג! לא, אלולב.

In any event, the Tosefta teaches that R' Yehuda says: Even dry species are fit for use in fulfilling the mitzva. What, is it not referring to an etrog as well, indicating that in his opinion an etrog does not require beauty? No, he was stating only that a dry lulav is fit for use.

אמר מר: כשם שאין פוחתין מהן, כך אין מוסיפין עליהן. פשיטא? מהו דתימא: הואיל ואמר רבי יהודה לולב צריך אגד, ואי מייתי מינא אחרינא, האי לחודיה קאי והאי לחודיה קאי, קא משמע לן.

The Master stated in the baraita cited above: Just as one may not diminish from their number, so too, one may not add to their number. The Talmud asks: That is obvious. Why would it be permitted to add an additional species? The Talmud answers: Lest you say: Since R' Yehuda said that a lulav requires a binding, and that requirement is a fundamental component of the mitzva, and if you bring another additional species, this species stands alone and that species stands alone, i.e., because the additional species is not bound with the others, its presence is insignificant, and causes no problem, therefore, R' Yehuda teaches us that this is not the case. In fact, one may not bring an additional species.

אמר מר: לא מצא אתרוג ⁠— לא יביא לא רמון ולא פריש ולא דבר אחר. פשיטא! מהו דתימא: לייתי, כי היכי שלא תשכח תורת אתרוג, קא משמע לן, זימנין דנפיק חורבא מיניה, דאתי למסרך.

The Master stated in the baraita cited above: If one cannot find an etrog, he may not bring a pomegranate, a quince, or anything else instead. The Talmud wonders: This is obvious. The Talmud answers: Lest you say: He should bring these fruits so that the halakhic category of the etrog will not be forgotten, therefore, R' Yehuda teaches us that it is in fact prohibited because on occasion, damage will result from this practice. Some may come to be drawn to this practice and use these species even when etrogim are available.