Musnad Ahmad 30:496

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ayyub, from Ibn Sirin, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "One of you remains in a state of prayer as long as he is waiting for the prayer. And the angels continue to pray for one of you as long as he is in his mosque, saying: 'O Allah, forgive him; O Allah, have mercy upon him.'"