Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) had only one Mu'adh-dhin. When he came out he would give the Adhan and when he came down (from the pulpit) he would give the Iqamah. Abu Bakr and Umar did likewise, but when Uthman (became caliph) the numbers of people had increased, he added the third call from atop a house in the marketplace that was called Zawra'. When he came out (the Mu'adh-dhin) would call the Adhan, and when he came down from the pulpit, he would call the Iqamah. Hasan