Sahih Bukhari Volume 001, Book 003, Hadith Number 058.
Narated By Ibn 'Umar : Allah's Apostle said, "Amongst the trees, there is a tree, the leaves of which do not fall and is like a Muslim. Tell me the name of that tree." Everybody started thinking about the trees of the desert areas. And I thought of the date-palm tree but felt shy to answer the others then asked, "What is that tree, O Allah's Apostle ?" He replied, "It is the date-palm tree."