(21) باب الأَذَانِ يَوْمَ الْجُمُعَةِ
Narrated As-Saib bin Yazid:
In the lifetime of the Prophet, Abu Bakr and `Umar, the Adhan for the Jumua prayer used to be pronounced when the Imam sat on the pulpit. But during the Caliphate of `Uthman when the Muslims increased in number, a third Adhan at Az-Zaura' was added. Abu `Abdullah said, "Az-Zaura' is a place in the market of Medina."
Reference: Sahih al-Bukhari 912
In-book reference: Book 11, Hadith 36
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 13, Hadith 35