Imam al-Zuhri (may Allah have mercy on him) narrates from Sa'ib bin Yazid (may Allah be pleased with him) that in the time of the Prophet (peace and blessings be upon him), Abu Bakr (may Allah be pleased with him) and Umar (may Allah be pleased with him), the first call was given when the imam came out, and the second when the prayer was established. In the time of Sayyiduna Uthman (may Allah be pleased with him) the number of people grew, so a third call was begun at the place called al-Zawra', and it has continued until now.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5681