Musnad Ahmad 30:312

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A man's prayer in congregation exceeds his prayer in his house and his prayer in his marketplace by twenty-some degrees. That is because when one of you performs ablution and perfects the ablution, then comes to the mosque, desiring nothing but the prayer, and nothing urges him except the prayer, he does not take a step except that a degree is raised for him by it, and a sin is removed from him by it, until he enters the mosque. So when he enters the mosque, he is in a state of prayer as long as the prayer is what detains him. And the angels pray upon one of them as long as he is in his place where he prayed, saying: 'O Allah, forgive him; O Allah, have mercy upon him; O Allah, accept his repentance,' as long as he does not cause harm in it, and as long as he does not commit a hadath (break his ablution) in it."