Hadith 500930
Chapter 2: Purity - كتاب الطهارة
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّهُ كَانَ يَقُولُ مَنْ مَسَّ ذَكَرَهُ فَقَدْ وَجَبَ عَلَيْهِ الْوُضُوءُ .
Yahya related to me from Malik from Hisham ibn Urwa that his father used to say, "If any of you touches his penis he has to do wudu."
Reference Muwatta Malik 500930
USC-MSA web (English) reference : Book 2, Hadith 63