Musnad Ahmad 27:1197

Hajjaj narrated to us, Shareek narrated to us, from Salamah ibn Kuhayl, from Mujahid, from Ibn Umar, from the Prophet, peace and blessings of Allah be upon him, regarding His saying: "{Like a good tree} [Ibrahim: 24]," he said: "It is the one that does not shed its leaves." And I thought that it was the date palm tree.