I think before the mimbar was made, the prophet [saw] used to stand up and hold on to a staff for support. A tree trunk I think, which then cried (made a crying sound of a camel) when the mimbar was made and was used for Sermons there after.
Allahu Alim.
Edit: From Bukhari - Book of Jummah. (#876)
It is related that Jabir ibn 'Abdullah was heard to say, "There was a palm trunk which the Prophet, may Allah bless him and grant him peace, used to lean against. When the minbar was made for him, we heard the trunk making a sound like a pregnant she-camel until the Prophet, may Allah bless him and grant him peace, came down and placed his hand on it."