Maliks Muwatta Book 5, Hadith Number 9.
Section : Paying Attention when the Imam is Giving the Khutba on the Day of Jumua.
Yahya related to me from Malik from Nafi that Abdullah ibn Umar saw two men talking while the imam was giving the khutba on the day of jumua and he threw pebbles at them to tell them to be quiet.