It is narrated from Abu Hurairah (may Allah be pleased with him) that he reports from the Messenger of Allah (peace and blessings be upon him) that he said: "When any one of you touches his private part with his hand while there is nothing (such as cloth, etc.) in between, then he should perform ablution."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 58
Hadith Takhrijأخرجه أحمد: 2/ 333 - وقال الشيخ الأرناؤوط: ”حسن“ والبيهقي: 133/1 وفي المعرفة السنن والآثار له (187) (188)، وصححه ابن حبان (1118).