Muwatta Malik, Book 2, Hadith 64 (USC-MSA)

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.' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، أَنَّهُ قَالَ رَأَيْتُ أَبِي عَبْدَ اللَّهِ بْنَ عُمَرَ يَغْتَسِلُ ثُمَّ يَتَوَضَّأُ فَقُلْتُ لَهُ يَا أَبَتِ أَمَا يَجْزِيكَ الْغُسْلُ مِنَ الْوُضُوءِ قَالَ بَلَى وَلَكِنِّي أَحْيَانًا أَمَسُّ ذَكَرِي فَأَتَوَضَّأُ ‏.‏

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also referenced under: Book 2, Hadith 94 (In-Book)