It is narrated from the Mother of the Believers Umm Habibah (may Allah be pleased with her) that she said: I heard the Prophet (peace and blessings be upon him) say: "Whoever touches his private part, let him perform ablution."
Al-Tirmidhi said: I asked Abu Zur'ah about the hadith of Umm Habibah, and he considered it good; I saw that he regarded it as mahfuz (preserved).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 626