For the source text click/tap here: Sukkah 36
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According to some sages, three of the species (all except the etrog) must be bound together. In the mishnah there is a debate whether the cord used to bind the three together must be from the same species as one of the three species. As we shall see in the Talmud, the problem with it being from another type of tree is that when he picks up the lulav, he will be carrying five species—the four mandated ones and the one from which he made his cord. This might be a violation of the prohibition of adding on to the Torah’s commandments. The Torah says four species—it would be prohibited to add a fifth.
The background of this dispute is Nehemiah 8:15, which is quoted below. In this verse Nehemiah and Ezra and the people who have returned to Israel after the first exile seem to interpret Leviticus 23:40 as if it mandates building the sukkah from the four species. However, their identification of the species slightly differs from the normative rabbinic interpretation and from the precise wording of Leviticus. Biblical scholars nevertheless interpret this verse as referring to building the sukkah from the four species.
Since the holiday is not called Sukkot in Nehemiah, but chag, the practice of taking four species is not mentioned, and the key requirement was to build a sukkah made of five species, it seems apparent that the holiday was named Sukkot after 430 BCE, when its practices and significance were changed, and the biblical mandates were placed in the Torah when these changes were made.
Recent scholars such as Ehrlich (a tragic figure unaccepted by both orthodox and reform) suggest an alternative historical development of the chag.