Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said: The first adhan used to be given when the imam sat on the pulpit on Friday. This was in the time of the Messenger of Allah (peace and blessings be upon him) and of Sayyiduna Abu Bakr and Umar. Then, when the caliphate of Sayyiduna Uthman (may Allah be pleased with him) came and the people had increased, Sayyiduna Uthman bin Affan (may Allah be pleased with him) ordered a third adhan on Friday, and it came to be given at al-Zawra'.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6618