Musannaf Ibn Abi Shayba 3:1693

Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you enters the mosque, he is in prayer as long as the prayer detains him. And the angels pray for one of you as long as he remains in his place where he prayed, saying: 'O Allah, forgive him; O Allah, have mercy on him; O Allah, accept his repentance,' as long as he does not cause harm in it, and as long as he does not break his ablution in it."