Musnad Ahmad 27:825

'Abd al-Malik ibn 'Amr narrated to us, Malik narrated to us, from 'Abd Allah ibn Dinar, from Ibn 'Umar, that the Prophet (peace and blessings of Allah be upon him) said: "What is a tree whose leaves do not fall, and it is like the believer?" — or he said: "The Muslim." He said: So the people began to guess the trees of the desert. Ibn 'Umar said: It occurred to me that it was the date palm. So the Messenger of Allah (peace and blessings of Allah be upon him) said: "It is the date palm." He said: So I mentioned that to 'Umar, and he said: "For you to have said it would have been more beloved to me than such and such."