Muwatta Malik, Book 2, Hadith 92 (In-Book) ⓘ
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "If you touch your penis you have to do wudu."
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا مَسَّ أَحَدُكُمْ ذَكَرَهُ فَقَدْ وَجَبَ عَلَيْهِ الْوُضُوءُ .
also referenced under: Book 2, Hadith 62 (USC-MSA)