Musnad Ahmad 30:3177

I read to Abd al-Rahman: Malik, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "The angels pray for one of you as long as he remains in his place of prayer, saying: 'O Allah, forgive him; O Allah, have mercy upon him.'"