The Messenger of Allah ﷺ said: "If you say to your companion: 'Be quiet and listen' when the imam is delivering the Khutbah, you have engaged in idle speech."  

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