سورة Al-A'raf (The Heights ) - الآية 204

So, when the Quran is recited, listen to it, and be silent that you may receive mercy. [i.e. during the compulsory congregational prayers when the Imam (of a mosque) is leading the prayer (except Surat Al-Fatiha), and also when he is delivering the Friday-prayer Khutbah]. [Tafsir At-Tabari, Vol. 9, Pages 162-4]