Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 182
Narated By Talq : We came upon the Prophet of Allah (pbuh). A man came to him: he seemed to be a bedouin. He said: Prophet of Allah, what do you think about a man who touches his penis after performing ablution? He (pbuh) replied: That is only a part of his body.
Abu Dawud said: This tradition has been narrated through a different chain of narrators.