Musnad Ahmad 154:13

Waki’ narrated to us, Ibn Abi Dhi’b narrated to us, from al-Zuhri, from al-Sa’ib ibn Yazid, who said: "The Adhan during the time of the Messenger of Allah (may Allah bless him and grant him peace), Abu Bakr, and ‘Umar (may Allah be pleased with them both) was two Adhans. Until it was the time of ‘Uthman, and the people became numerous, so he ordered the first Adhan at al-Zawra’."