The Messenger of Allah ﷺ said, "If you tell your companion 'Be quiet' while the imam is delivering the sermon, you have spoken idly."
حدثنا القعنبي عن مالك عن ابن شهاب عن سعيد
أن رسول الله ﷺ قال إذا قلت أنصت والإمام يخطب فقد لغوت
[Sahih] Narrated by Nasai in "Prayer of the Two Eids," chapter "Silence during the sermon," no. 1578, from the hadith of Malik through this chain, and it is found in Muwatta, narration of Abdul-Rahman bin Qasim, no. 13. It was also narrated by Bukhari, no. 934, and Muslim, no. 851, from the hadith of Ibn Shihab Zuhri through this chain.