Anas bin Malik (May Allah be pleased with him) reported:
When we were in Al-Madinah, the moment the Mu'adhdhin finished the Adhan of the Maghrib prayer, the people hastened to the pillars of the mosque and performed two Rak'ah prayer behind them. A stranger coming into the mosque would think that the obligatory prayer had already been performed because of the number of people performing them.

[Muslim].

وعنه قال: كنا بالمدينة فإذا أذن المؤذن لصلاة المغرب ابتدروا السواري، فركعوا ركعتين، حتى إن الرجل الغريب ليدخل المسجد فيحسب أن الصلاة قد صليت من كثرة من يصليهما. ((رواه مسلم)).