Musnad Ahmad 30:2998

Yahya narrated to us, from Malik, who said: Al-Zuhri narrated to me, from Sa'id ibn al-Musayyib, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "If a man says to his companion on the day of Friday, while the Imam is delivering the sermon: 'Be silent,' then he has engaged in idle talk."