Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Urwah bin Zubair (may Allah have mercy on him) that I went to Marwan bin Al-Hakam (the Umayyad), and we discussed what things make ablution (wudu) necessary. Marwan said: Touching the private part (penis) makes ablution necessary. Urwah said: I do not know about this. Marwan said: Busrah bint Safwan (may Allah be pleased with her) informed me that she heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "When any one of you touches his private part, he should perform ablution."