The Prophet (peace and blessings be upon him) said:
"When any of you extends his hand to his private part (and touches it), he should perform ablution."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 361
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ن] عن بسرة بنت صفوان. الصحيحة *١٢٣٥: ك.