Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 169
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 169, with English translation by Maimonides (Rambam).
ליטול לולב, שנאמר: "ולקחתם לכם ביום הראשון, פרי עץ הדר כפות תמרים" (ויקרא כג,מ).
To take the lulav [and the other three species on Sukkot], as [Leviticus 23:40] states: "And you shall take for yourselves on the first day, the fruit of a beautiful tree, a palm branch,...."