Messenger of Allah ﷺ 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."  

أَخْبَرَهُ أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا قُلْتَ لِصَاحِبِكَ يَوْمَ الْجُمُعَةِ أَنْصِتْ وَالإِمَامُ يَخْطُبُ فَقَدْ لَغَوْتَ