Musnad Ahmad 30:433

Ya'la narrated to us, Muhammad ibn Ishaq narrated to us, from al-'Ala' ibn 'Abd al-Rahman, from his father, from Abu Hurayrah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "When one of you prays, then sits in his place of prayer, the angels do not cease saying: 'O Allah, forgive him; O Allah, have mercy upon him,' as long as he does not commit a hadath (break his ablution) or stand up."