Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said: Sayyiduna Uthman (may Allah be pleased with him) ordered the third adhan on Friday when the number of the people of Madinah had grown, whereas the Messenger of Allah (peace and blessings be upon him) had only one mu'adhdhin, and the adhan on Friday was given only when the imam sat down.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6619