Sukkah 37A

לא מצא ארבעת מינין יהא יושב ובטל, והתורה אמרה: ״בסוכות תשבו שבעת ימים״⁠ — סוכה של כל דבר. וכן בעזרא אומר: ״צאו ההר והביאו עלי זית ועלי עץ שמן ועלי הדס ועלי תמרים ועלי עץ עבות (ועשו) סוכות ככתוב״!

According to your reasoning, if one did not find any of the 4 species to roof his sukka, he will sit idly and fail to fulfill the mitzva of sukka; and the Torah states: “You should reside in sukkot for 7 days” (Leviticus 23:42), meaning a sukka of any material. Likewise, in the book of Ezra, which can refer also to the book of Nehemiah, it says: “Go forth to the mount, and fetch olive branches, and pine branches, and myrtle branches, and palm branches, and branches of a dense-leaved tree, to make sukkot, as it is written” (Nehemiah 8:15). Apparently, a sukka may be constructed even with materials other than the 4 species.

ורבי יהודה סבר: הני ⁠— לדפנות, עלי הדס ועלי תמרים ועלי עץ עבות ⁠— לסכך. ותנן: מסככין בנסרין, דברי רבי יהודה. אלמא סיב ועיקרא דדיקלא מינא דלולבא הוא, שמע מינה.

And R' Yehuda holds: These olive branches and pine branches mentioned in the verse were for the walls of the sukka, which need not be built from the 4 species. Myrtle branches, palm branches, and branches of a dense-leaved tree, i.e., again myrtle, all of which are among the 4 species, were for the roofing. R' Yehuda holds that one may roof the sukka only with the 4 species. And we learned in a Mishnah: One may roof the sukka with boards; this is the statement of R' Yehuda. As boards can be produced from one of the 4 species only if the trunk of the date palm is considered a lulav, apparently, fibers and the trunk of the date palm are the species of the lulav. The Talmud determines: Indeed, conclude from it that this is so.