Musnad Ahmad 30:3170

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "If you say to your companion while the Imam is delivering the sermon, 'Be silent,' then you have engaged in idle talk."