Sayyidah Busrah bint Safwan (may Allah be pleased with her) narrated that she heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever touches his private part, let him perform ablution."
Urwah used to say: "Whoever touches the groin, or the testicles, or the private part, let him perform ablution."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 653