Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said: The first adhan on Friday in the time of the Messenger of Allah (peace and blessings be upon him) was that when the Messenger of Allah (peace and blessings be upon him) sat on the pulpit, the adhan would be given, and when he (peace and blessings be upon him) came down, the iqamah would be called. This practice continued in the time of the Prophet (peace and blessings be upon him) and of Sayyiduna Abu Bakr and Umar (may Allah be pleased with them both). Then, when the time of Sayyiduna Uthman (may Allah be pleased with him) came and the people had spread out and increased, he appointed a mu'adhdhin who would give the adhan at al-Zawra' before he came out.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6614