Yahya related to me from Malik that Abdullah ibn Abi Bakr ibnMuhammad ibn Amr ibn Hazim heard Urwa ibn az-Zubayr saying, "I went tosee Marwan ibn al-Hakam and we talked about what you had to do wudufor, 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 Busrabint Safwan had told him that she heard the Messenger of Allah, mayAllah bless him and grant him peace, say, 'If any of you touches hispenis he should do wudu.' "
USC-MSA web (English) reference: Book 2, Hadith 60
Arabic reference: Book 2, Hadith 90