Musnad Ahmad 30:3757

Hammad narrated to us, from Malik and Ibn Abi Dhi'b, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "If you say to your companion while the Imam is delivering the sermon on Friday, 'Listen silently,' then you have engaged in idle talk."