Musannaf Ibn Abi Shayba 2:194

Ibn Idris narrated to us, from Muhammad ibn Ishaq, from al-Zuhri, from al-Sa’ib ibn Yazid, the nephew of Nimr, who said: The Messenger of Allah (may Allah bless him and grant him peace) had only one caller (mu’adhdhin) who would call the adhan when he sat upon the pulpit, and would perform the iqamah when he descended. Then Abu Bakr was the same, then ‘Umar was the same, until it was the time of ‘Uthman and the people spread out and increased in number. He added the third call at the time of the sun’s meridian (al-zawāl) or at al-Zawra’.