Musannaf Ibn Abi Shayba 3:2919

'Abbad ibn al-'Awwam narrated to us, from Yahya ibn Sa'id, from Yazid ibn 'Abd Allah, from Tha'labah ibn Malik al-Qurazi, who said: I reached 'Umar and 'Uthman. When the Imam would come out on the day of Jumu'ah, we would leave off the prayer. And when he would speak, we would leave off speaking.