Musnad Ahmad 27:1592

Hashim and Hujayn narrated to us, they said: Abd al-Aziz narrated to us, from Abdullah ibn Dinar, from Ibn Umar, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The likeness of the believer is like a tree that does not shed its leaves." He said: People began to think of the trees of the desert, and it occurred in my heart that it was the date palm, but I was shy to speak. Then the Messenger of Allah, peace and blessings of Allah be upon him, said: "It is the date palm." He said: I mentioned that to Umar, and he said: "O my son, what prevented you from speaking? For by Allah, that you had said that would have been more beloved to me than having such and such."