It is narrated from Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) that on Friday the adhan began when the imam sat down, and when the prayer was established. This was in the time of the Messenger of Allah (peace and blessings be upon him) and of Sayyiduna Abu Bakr and Umar (may Allah be pleased with them both), until, when the time of Sayyiduna Uthman (may Allah be pleased with him) came and the people had increased, he added a third adhan at al-Zawra'.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6617