I heard As-Saib bin Yazid, saying, "In the lifetime of Allah's Messenger (ﷺ), and Abu Bakr and `Umar, the Adhan for the Jumua prayer used to be pronounced after the Imam had taken his seat on the pulpit. But when the people increased in number during the caliphate of `Uthman, he introduced a third Adhan (on Friday for the Jumua prayer) and it was pronounced at Az-Zaura' and that new state of affairs remained so in the succeeding years.
Sahih al-Bukhari Book 11, Hadith 40
← Previous Hadith `Uthman bin `Affan introduced the second Adhan on Fridays when the number of the people in the mosque increased. Previously the Adhan on Fridays used to be pronounced only after the Imam had taken his seat (on the pulpit).

Sahih al-Bukhari Book 11, Hadith 39 Next Hadith → Some people went to Sahl bin Sa`d As-Sa`idi and told him that they had different opinions regarding the wood of the pulpit. They asked him about it and he said, "By Allah, I know of what wood the pulpit was made, and no doubt I saw it on the very first day when Allah's Messenger (ﷺ) took his seat on it. Allah's Messenger (ﷺ) sent for such and such an Ansari woman (and Sahl mentioned her name) and said to her, 'Order your slave-carpenter to prepare for me some pieces of wood (i.e. pulpit) on which I may sit at the time of addressing the people.' So she ordered her slave-carpenter and he made it from the tamarisk of the forest and brought it (to the woman). The woman sent that (pulpit) to Allah's Messenger (ﷺ) who ordered it to be placed here. Then I saw Allah's Messenger (ﷺ) praying on it and then bowed on it. Then he stepped back, got down and prostrated on the ground near the foot of the pulpit and again ascended the pulpit. After finishing the prayer he faced the people and said, 'I have done this so that you may follow me and learn the way I pray.' "

Sahih al-Bukhari Book 11, Hadith 41