Maliks Muwatta Book 2, Hadith Number 64.
Section : Wudu on Account of Touching the Genitals.
Yahya related from Malik from Ibn Shihab that Salim ibn Abdullah said, “I saw my father Abdullah ibn Umar, do ghusl and then do wudu. I said to him, ‘Father, isn’t ghusl enough for you?’ He said, ‘Of course, but sometimes I touch my penis, so I do wudu.'”