Busrah narrates that the Prophet Muhammad (peace be upon him) said: "Whoever touches his private part should perform ablution again." (Imam Ibn Hibban, may Allah have mercy on him, says:) If what was meant was to wash both hands, as some people have said, then the Prophet (peace be upon him) would not have said: "Perform ablution again," because only that ablution can be repeated which is done for prayer.
Hadith Referenceصحیح ابن حبان / كتاب الطهارة / 1115
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرطهما