وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا مَسَّ أَحَدُكُمْ ذَكَرَهُ فَقَدْ وَجَبَ عَلَيْهِ الْوُضُوءُ ‏.‏

Yahya related to me from Malik from Nafi that Abdullah ibn Umarused to say, "If you touch your penis you have to do wudu." Translation Not Available

Book Ref: Muwatta Imam Malik Book 2 Hadith 92
Web Ref:  Muwatta Imam Malik Book 2 Hadith 62