Sunan An-Nasai, Chapter: The Book of Purification

"When he was the governor of Al-Madinah, Marwan mentioned that a man should perform Wudu after touching his penis, if he touches it with his hand. I did not like that and I said: 'The one who touches it does not have to perform Wudu.' Marwan said: 'Busrah bint Safwan told me that she heard the Messenger of Allah (saw) mention the things for which Wudu should be performed, and the Messenger of Allah (saw) said: 'Wudu should be performed after touching the penis.' 'Urwah said: 'I continued to argue with Marwan until he called one of his guards and sent him to Busrah to ask her about what Marwan had narrated, and Busrah sent word saying something like that which Marwan had narrated to me from her." (Sahih)