Sahih Muslim Book - 7, Hadith - 1986

وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا يَعْقُوبُ - يَعْنِى ابْنَ عَبْدِ الرَّحْمَنِ - عَنْ سُهَيْلٍ عَنْ أَبِيهِ عَنْ أَبِى هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- قَالَ « عَلَى كُلِّ بَابٍ مِنْ أَبْوَابِ الْمَسْجِدِ مَلَكٌ يَكْتُبُ الأَوَّلَ فَالأَوَّلَ - مَثَّلَ الْجَزُورَ ثُمَّ نَزَّلَهُمْ حَتَّى صَغَّرَ إِلَى مَثَلِ الْبَيْضَةِ - فَإِذَا جَلَسَ الإِمَامُ طُوِيَتِ الصُّحُفُ وَحَضَرُوا الذِّكْرَ ».

It was narrated from Abu Hurairah that the Messenger of Allah (s.a.w) said: "At every door of the Masjid there stands an Angel who writes down the first one to come, then the next." He likened them to (the one who offered) a camel then in descending order, to (the one who offered) an egg. "Then when the Imam sits down, the records are closed and they (the Angels) attend the reminder."