Musnad Ahmad 30:1998

Abu Ahmad narrated to us, Kathir bin Zayd narrated to us, from al-Walid bin Rabah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "One of you remains in a state of prayer as long as he is in his place of sitting, waiting for the prayer. And the angels say: 'O Allah, forgive him; O Allah, have mercy upon him,' as long as he does not commit a hadath (break his ablution)."