Muwatta Imam Malik Book of Purity, Hadith 63

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّهُ كَانَ يَقُولُ مَنْ مَسَّ ذَكَرَهُ فَقَدْ وَجَبَ عَلَيْهِ الْوُضُوءُ .