Sahih Muslim Book - 6, Hadith - 1939

وَحَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ حَدَّثَنَا عَبْدُ الْوَارِثِ عَنْ عَبْدِ الْعَزِيزِ - وَهُوَ ابْنُ صُهَيْبٍ - عَنْ أَنَسِ بْنِ مَالِكٍ قَالَ كُنَّا بِالْمَدِينَةِ فَإِذَا أَذَّنَ الْمُؤَذِّنُ لِصَلاَةِ الْمَغْرِبِ ابْتَدَرُوا السَّوَارِىَ فَيَرْكَعُونَ رَكْعَتَيْنِ رَكْعَتَيْنِ حَتَّى إِنَّ الرَّجُلَ الْغَرِيبَ لَيَدْخُلُ الْمَسْجِدَ فَيَحْسِبُ أَنَّ الصَّلاَةَ قَدْ صُلِّيَتْ مِنْ كَثْرَةِ مَنْ يُصَلِّيهِمَا.

It was narrated that Anas bin Malik said: "We were in Al-Madinah and when the Mu'adhdhin called the Adhan for Magbrib prayer, they would hasten to the pillars and pray two Rak'ah, until if a stranger were to enter the Masjid, he would think that the prayer had begun, because of the large numbers of people who prayed them."