Musnad Abu Ya'la al-Mawsili 160:68

Abu Khaythama narrated to us, Jarir narrated to us, from Muhammad ibn Ishaq, from al-Ala’ ibn Abd al-Rahman, from his father, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him) that he used to say: "When he prays, then sits in his place of prayer where he prayed, the angels do not cease to pray for him: '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."