Maliks Muwatta Book 2, Hadith Number 113.
Section : Urinating Standing and Otherwise.
Yahya related to me from Malik that Yahya ibn Said said, “A Bedouin came into the mosque and uncovered his private parts to urinate. The people called out to him and began to raise their voices but the Messenger of Allah, may Allah bless him and grant him peace, said,’Let him be. ‘So they let him be and he urinated. Then the Messenger of Allah, may Allah bless him and grant him peace, ordered a bucketful of water to be brought and it was poured on the place.”