Qays bin Talq narrates from his father, Sayyiduna Talq bin Ali (may Allah be pleased with him), that he said: I came to the Prophet (peace and blessings be upon him) while he was building the mosque, and he (peace and blessings be upon him) said: "Mix the clay, for you know best how to do this." I asked him (peace and blessings be upon him), or someone else asked: What is your (peace and blessings be upon him) view about a man who performs ablution and then touches his private part? He (peace and blessings be upon him) said: "It is only a part of your own body."
Then some of our companions interpreted it as touching it with the back of the hand.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 646