Musnad Ahmad 30:3770

Muhammad ibn Isma'il ibn Abi Fudayk narrated to us, al-Dahhak ibn 'Uthman narrated to us, from al-Maqburi, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, the believing servant is in a state of prayer as long as he is in his place of prayer, with nothing detaining him except the waiting for the prayer. And the angels are with him, saying: 'O Allah, have mercy upon him; O Allah, forgive him,' as long as he does not commit a hadath (break his ablution)."