Sahih Bukhari Volume 001, Book 003, Hadith Number 133.
Narated By 'Abdullah bin 'Umar : Once 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 asked, "O Allah's Apostle! inform us of it." He replied, "it is the date-palm tree." I told my father what had come to my mind and on that he said, "Had you said it I would have preferred it to such and such a thing that I might possess."