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He reported God's Messenger as saying, “When you tell your companion on Friday to be silent while the imam is preaching, you are guilty of idle talk." (Bukhari and Muslim.)
Source: sunnah.com
Other translations
Arabic
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " إِذَا قُلْتَ لِصَاحِبِكَ يَوْمَ الْجُمُعَةِ أنصت وَالْإِمَام يخْطب فقد لغوت)
sunnah.com