Musnad Ahmad 27:411

Yazid ibn Harun narrated to us, Shu'bah informed us, from Muharib ibn Dithar, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the likeness of the believer is like a tree whose leaves do not fall. So what is it?" He said: They said this and they said that, but they did not hit the mark. I wanted to say: "It is the date palm," but I felt shy. Then the Prophet (peace and blessings of Allah be upon him) said: "It is the date palm."