Musnad Ahmad 30:3368

Yazid narrated to us, Muhammad—meaning Ibn Ishaq—informed us, from al-Ala’ ibn Abd al-Rahman ibn Ya’qub, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you prays and 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 stand up or commit a hadath (break his ablution)."