Purification (Kitab Al-Taharah)
Dawud :: Book 1 : Hadith 182
Narrated Talq:
We came upon the Prophet of Allah (peace_be_upon_him). 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 (peace_be_upon_him) replied: That is only a part of his body.