Musannaf Ibn Abi Shayba 34:192

Isma'il—meaning Ibn 'Ulayyah—narrated to us, from Dharr, from al-Zuhri: The Adhan was at the time of the Imam's exit. Then the Commander of the Faithful, Uthman, introduced the second Adhan upon al-Zawra' in order to gather the people.