Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.143.

Tha’labah ibn abi-Malik says: “We would be talking on Jumu’ah while ‘Umar was sitting on the pulpit and when the call to salah was finished ‘Umar would stand and no one would utter a word until he had completely finished both of his khutbahs. When the iqamah was made and ‘Umar came down from the pulpit, the people would then speak.” This is related by ash-Shaf’i in his Musnad.

Ahmad records, with a sahih chain, that while the azhan was being made, ‘Uthman ibn-‘Affan would be sitting on the pulpit, apprising the people of their situation and the prices of some commodities.