The Messenger of Allah ﷺ said, "When one of you touches his private part, he must perform ablution."
حدثنا محمد بن عبد الله بن نمير حدثنا عبد الله بن إدريس عن هشام بن عروة عن أبيه عن مروان بن الحكم
قال رسول الله ﷺ إذا مس أحدكم ذكره فليتوضأ
Its isnad is hasan. Tirmidhi reported it in the chapter on wudu, in the section on wudu after touching the private part, 83, from the hadith of Hisham through this route, and he said, "Hasan sahih." See also Sunan Abu Dawud, 181, with my commentary Nail al-Maqsud.