Musnad Ahmad 27:1427

Husayn narrated to us, Khalaf narrated to us, from Abu Janab, from his father, from Abdullah ibn Umar, who said: There was a date-palm trunk in the mosque against which the Messenger of Allah, peace and blessings of Allah be upon him, would lean his back when it was a Friday, or when an affair occurred about which he wanted to speak to the people. They said: "O Messenger of Allah, shall we not make for you something to the extent of your standing?" He said: "No, it is not upon you to do so." Then they made for him a pulpit with three steps. He said: So he sat upon it. He said: The trunk groaned like a cow groans out of grief for the Messenger of Allah, peace and blessings of Allah be upon him. So he embraced it and wiped it until it became still.