Musnad Ahmad 5:94

Waki’ narrated to us, ‘Abd al-Malik ibn Muslim al-Hanafi narrated to us, from his father, from ‘Ali, who said: A Bedouin came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, we are in the desert and one of us passes a little wind." The Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, Allah the Almighty and Majestic is not ashamed of the truth. If one of you does that, let him perform ablution, and do not approach women in their posteriors." And he said another time: "In their anuses."