Sayyidah Aishah Siddiqah (may Allah be pleased with her) narrates that the Prophet (peace and blessings be upon him) said: "Whoever touches his private part should perform ablution."
Al-Bazzar recorded it; its chain contains a narrator, Umar bin Shurayh. Al-Azdi says: The hadith he narrates is not authentic.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1269