Musnad Ahmad 30:2335

Harun narrated to us, he said: Ibn Wahb narrated to us, he said: Yunus informed me, from Ibn Shihab, from Ibn Hurmuz, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, one of you is in prayer as long as he sits waiting for the prayer, as long as he does not break his ablution. The angels pray for him: 'O Allah, forgive him; O Allah, have mercy on him.'"