Abu Dawud Book 1, Hadith Number 182.

Chapter : Ablution is not necessary after touching the penis.

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.