Yahya related from Malik from Ibn Shihab that Salim ibn Abdullahsaid, "I saw my father Abdullah ibn Umar, do ghusl and then do wudu. Isaid to him, 'Father, isn't ghusl enough for you?' He said, 'Ofcourse, but sometimes I touch my penis, so I do wudu.' "
USC-MSA web (English) reference: Book 2, Hadith 64
Arabic reference: Book 2, Hadith 94