Sunan an-Nasa'i, Volume 2, Book 14, Hadith 1402 (DarusSalam)

It was narrated from Abu Hurairah that: The Messenger of Allah said: "Whoever says to his companion on Friday, when the imam is delivering the khutbah: 'Listen attentively,' has engaged in idle talk."

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ الزُّهْرِيِّ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ مَنْ قَالَ لِصَاحِبِهِ يَوْمَ الْجُمُعَةِ وَالإِمَامُ يَخْطُبُ أَنْصِتْ فَقَدْ لَغَا ‏" ‏ ‏.‏

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also referenced under: Book 14, Hadith 1412 (In-Book)