Hadith 121
قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " إِذَا قُلْتَ لِلنَّاسِ أَنْصِتُوا وَهُمْ يَتَكَلَّمُونَ، فَقَدْ لَغَوْتَ عَلَى نَفْسِكَ يَعْنِي يَوْمَ الْجُمُعَةِ "
And the Messenger of Allah, peace and blessings be upon him, said: "On the day of Friday (during the sermon), your saying to the people 'Be quiet' while they are talking among themselves is an idle and vain thing."