Sayyiduna Talq bin Ali (may Allah be pleased with him) said: A man asked the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, tell us: when one of us touches his private part, should he perform ablution? He (peace and blessings be upon him) said: "No, it is only a piece of your own body."
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8218