It was narrated from Abu Hurairah that:
The Messenger of Allah (ﷺ) said: "Whoever says to his companion on Friday, when the imam is delivering the khutbah: 'Listen attentively,' has engaged in idle talk." (DE)
App reference (DE): Book 14, Hadith 38 (DE)
English reference (DE): Vol. 2, Book 14, Hadith 1402 (DE)
Arabic reference (DE): Book 14, Hadith 1412 (DE)