صحيح البخاري

Narrated Az-Zuhri:

I heard As-Saib bin Yazid, saying, "In the lifetime of Allah's Messenger, 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.