Abu Dawud Book 1, Hadith Number 181.

Chapter : By touching the penis ablution becomes void.

Narated By Busrah daughter of Safwan : Abdullah ibn Abu Bakr reported that he heard Urwah say: I entered upon Marwan ibn al-Hakam. We mentioned things that render the ablution void. Marwan said: Does it become void by touching the penis? Urwah replied: This I do not know. Marwan said: Busrah daughter of Safwan reported to me that she heard the Messenger of Allah (PBUH) say: He who touches his penis should perform ablution.