It is narrated from Sayyiduna Abu Ayyub (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Ablution should be performed after touching the private part," and sometimes he said: "Whoever touches his private part, let him perform ablution."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3913