One should keep quiet and listen while the Imam is delivering the Khutba

وَإِذَا قَالَ لِصَاحِبِهِ أَنْصِتْ. فَقَدْ لَغَا. وَقَالَ سَلْمَانُ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُنْصِتُ إِذَا تَكَلَّمَ الإِمَامُ.And if a person says to his companion: "Be quiet and listen," even then he is doing something wrong (which will reduce his reward). Narrated Salman: The Prophet ﷺ said, "Whenever the Imam is delivering the Khutba (religious talk), it is essential for the audience to keep quiet and listen." ​​