Urwah narrated that he was with Marwan bin al-Hakam when he was asked about touching the private organ, and he saw no harm in it. So Marwan sent one of his guards to Busrah bint Safwan (may Allah be pleased with her), and (Marwan) said: She told me that the Prophet (peace and blessings be upon him) said: "When a man touches his private parts with his hand, let him not pray until he performs ablution." He (the guard) returned and said: She said: Yes. Hisham said: After that my father (Urwah) used to say: Whoever touches his groin or his testicles, let him perform ablution.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20877