הלכה על סוכה 69:10

The commandment of taking the lulav: The commandment of lulav, that we should take in our hands on the first day of the holiday of Sukkot the fruit of a hadar tree, palm fronds, the branches of a braided tree and willows of a brook, as it is stated (Leviticus 23:40), "And you shall take for yourselves on the first day the fruit of a hadar tree and palm fronds, the branches of a braided tree and willows of a brook." And the explanation came about it (Sukkah 35a) that the fruit of the hadar tree is the citron (etrog); the fronds (kappot) of date palms is the lulav - and it is written kappat, lacking [a letter] vav (which could make it singular), to hint that the obligations is that we take one lulav, and not two or three or more; the branch of a braided tree is the myrtle; and the willows of a brook is the willow that is well-known among Israelites.