Muwatta Malik #90

Yahya related to me from Malik that Abdullah ibn Abi Bakr ibn Muhammad ibn Amr ibn Hazim heard Urwa ibn az-Zubayr saying, "I went to see Marwan ibn al-Hakam and we talked about what you had to do wudu for, and Marwan said, 'You have to do wudu if you touch your penis.' Urwa said, 'I didn't know that.' Marwan ibn al-Hakam said that Busra bint Safwan had told him that she heard the Messenger of Allah, may Allah bless him and grant him peace, say, 'If any of you touches his penis he should do wudu.' "