Is it sunnah for the Imam to read Khutbah of Jummah standing on the mimbar and what is the reference ?

This is sunnah that a person should stand on the pulpit and recite khutba. Khutba is mandatory. To recite it while on the pulpit is sunnah because the prophet peace be upon him had a built pulpit for him on which he delivered his khutba all the time.
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