Sahih Bukhari Volume 2, Book 13, Hadith Number 39

Narated By Az-Zuhri : I heard As-Saib bin Yazid, saying, “In the life-time of Allah’s Apostle, 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.