And you shall take for yourself on the first day the fruit of a (citron) beauteous tree (Leviticus 23:40).
Rabbi Mordechai of Nesh'chiz looked forward all year to the mitzvah of the Four Species on Succos. Since a fine esrog was costly and Rabbi Mordechai was hardly a man of means, he would accumulate small coins all year round, even depriving himself of food, in order to be able to afford an esrog.
A few days before Succos, Rabbi Mordechai joyfully took the money he had saved, and in high spirits, went off to buy the coveted esrog. On the way, he encountered a man sitting at the side of the road, weeping bitterly. He inquired as to the reason for the man's grief, and the latter told him, "Woe is to me! I earn my living with my horse and wagon, and this morning my nag died. How am I to feed my wife and children?"
"How much do you need to buy another horse?" Rabbi Mordechai asked.
The sum that the man specified was exactly the amount that Rabbi Mordechai had laboriously saved all year long for the esrog. Without giving it another thought, he gave his purse to the man. "Here, my dear man. Go buy yourself a horse'
After the man joyfully left with the money, Rabbi Mordechai said, "Oh well. All of Israel will be fulfilling the mitzvah of the Four Species with an esrog, but I will do so with a horse."
Rabbi Mordechai's sacrifice of his personal comfort all year round teaches us how precious is the mitzvah of the Four Species, but his final act teaches us that the mitzvah of tzedakah (charity) is even greater.