Musnad Ahmad 30:2251

'Affan narrated to us, Hammad narrated to us, Thabit informed us, from Abu Rafi', from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The servant remains in prayer as long as he is in his place of prayer, waiting for the prayer. The angels say: 'O Allah, forgive him; O Allah, have mercy on him,' until he leaves or breaks his wudu." I (the narrator) said: "And what is breaking his wudu?" He said: "Passing wind or breaking wind."