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 (may Allah be pleased with them both).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6621