Sahih Al Bukhari – Book of Friday Prayer

Volumn 002, Book 013, Hadith Number 056.
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Narated By Abu Huraira : Allah's Apostle (p.b.u.h) said, "When the Imam is delivering the Khutba, and you ask your companion to keep quiet and listen, then no doubt you have done an evil act."

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What i understood is you must ensure that you listen the khutba carefully and not even say a word to someone…

If someone talks duirng the khutba, teach him later so that he doesnt repeat the mistake in next friday.

Allah SWT knows the best