Musnad Ahmad 5:598

Abdullah narrated to us, Muhammad ibn Bakkar narrated to us, Habban ibn Ali narrated to us, from Dirar ibn Murrah, from Husayn al-Muzani, who said: Ali ibn Abi Talib said upon the pulpit: "O people, I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Nothing breaks the prayer except the *hadath' (ritual impurity)." I do not feel shy before you regarding that which the Messenger of Allah, peace and blessings of Allah be upon him, did not feel shy. He said: And the *hadath* is to pass wind silently or to pass wind audibly.