Musnad Ahmad 30:2341

Muhammad ibn Ubayd narrated to us, he said: Muhammad—meaning 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, saying: "When one of you prays, then sits, the angels do not cease saying: 'O Allah, forgive him; O Allah, have mercy upon him,' as long as he does not break his ablution or stand up."