Musannaf Ibn Abi Shayba 3:2917

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah, that the Prophet (may Allah bless him and grant him peace) said: "Whoever says to his companion on the day of Friday, 'Be silent,' while the Imam is delivering the sermon, has engaged in idle talk."