Saaib bin Yazid reported,
During the time of the Messenger of Allah ﷺ, Abu Bakr, and Umar, the call to prayer would be made when the imam came out and when the prayer was about to begin. When Uthman 
became caliph, he added a third call from atop az-Zawra.
Tirmidhi 516
Sahihحدثنا أحمد بن منيع حدثنا حماد بن خالد الخياط عن ابن أبي ذئب عن الزهري
عن السائب بن يزيد قال
كان الأذان على عهد رسول الله ﷺ وأبي بكر وعمر إذا خرج الإمام وإذا أقيمت الصلاة فلما كان عثمان رضي الله عنه زاد النداء الثالث على الزوراء
Sourcing, Bukhari also narrated it in the Book of Friday Prayer, chapter on the adhan on Friday, hadith no. 912, from the hadith of Ibn Abu Dhib with the same wording. This adhan was given at the pulpit, as is recorded in Mujam al-Kabir by Tabarani with a sahih chain, from Sulayman Taymi from Zuhri from Saaib bin Yazid with the same wording.