Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) says that on Friday, in the time of the Prophet (peace and blessings be upon him), Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them), the first adhan was given when the imam sat on the minbar. When the caliphate of Sayyiduna Uthman (may Allah be pleased with him) came and the number of people increased, he ordered the third adhan, which was given at the place called al-Zawra'; then the practice remained like that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5744