Musannaf Ibn Abi Shayba 3:3013

Ibn ‘Ulayyah narrated to us, from Burayd, who said: I said to al-Zuhri, "When do buying and selling become forbidden on the day of Jumu'ah?" He said: "The Adhan used to be at the time the Imam would come out. Then the Commander of the Believers, Uthman, introduced the third Adhan, and he had it called out from al-Zawra’ so that the people would gather. Therefore, I am of the opinion that buying and selling should be abandoned at [the time of] that Adhan."