Silence during the khutbah

Even saying "Be quiet" during the khutbah is idle talk.

Narrated by Abu Hurairah

If you say to your companion "Be quiet" on Friday while the imam is giving the khutbah, you have engaged in idle talk.

Sahih al-Bukhari 934, Sahih Muslim 851

What this hadith teaches

The khutbah is part of the Friday prayer. Listening in silence is required; even a gesture is better than words.

Reference and grade

Grade
Sahih: agreed upon by al-Bukhari and Muslim
Narrator
Abu Hurairah

The hadith in Arabic نص الحديث

إِذَا قُلْتَ لِصَاحِبِكَ يَوْمَ الْجُمُعَةِ أَنْصِتْ‏.‏ وَالإِمَامُ يَخْطُبُ فَقَدْ لَغَوْتَ

Arabic text of Sahih al-Bukhari 934, as given on sunnah.com.