Sayyiduna Sa’ib bin Yazid (may Allah be pleased with him) narrates that during 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 would come (to the mosque), and the second adhan was given when the congregation stood (i.e., the iqamah). During the time of Sayyiduna Uthman (may Allah be pleased with him), when the population increased, he ordered a third adhan to be given at the place called Zawra, and thus it continues to this day.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصلاة / 290