Musannaf Ibn Abi Shayba 3:2946

'Abdullah ibn al-Mubarak narrated to us, from Ibn Jurayj, from 'Ata', and from Ibn al-Mubarak, from al-Hasan, and from Ibn 'Alathah, from al-Zuhri, regarding the one who speaks while the Imam is delivering the sermon on Friday: They said, "He should pray two rak'ahs."