Qais bin Talq narrates from his father: We came to the service of the Prophet Muhammad (peace be upon him) as a delegation. A man came and asked: O Messenger of Allah! What do you say about a person who touches his private part after performing ablution? The Prophet Muhammad (peace be upon him) said: It (the private part) is a piece of flesh from his body (the narrator is in doubt, perhaps these are the words) a piece.
Hadith Referenceصحیح ابن حبان / كتاب الطهارة / 1119
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (176).فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده قوي.