Musnad Ahmad 30:3702

'Abd al-Samad narrated to us, Hammad narrated to us, from Thabit, from Abu Rafi', from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The servant remains in prayer as long as he is waiting for the prayer. The angels say: 'O Allah, forgive him; O Allah, have mercy upon him,' as long as he does not depart or break his wudu." It was said to him: "What is breaking his wudu?" He said: "He passes wind silently or audibly."