I entered upon Marwan bin Hakam, and we discussed what requires ablution. Marwan said, "Touching the penis requires ablution."Urwa said, "I did not know this."Marwan bin Hakam replied, "Busra bint Safwan informed me that she heard the Messenger of Allah ﷺ say, 'When one of you touches his penis, let him perform ablution.'"
دخلت على مروان بن الحكم، فتذاكرنا ما يكون منه الوضوء، فقال مروان: ومن مس الذكر الوضوء، فقال عروة: ما علمت هذا، فقال مروان بن الحكم: أخبرتني بسرة بنت صفوان أنها سمعت رسول الله ﷺ يقول: إذا مس أحدكم ذكره فليتوضأ.
Malik narrated it in Taharah (58) from Abdullah bin Abu Bakr bin Muhammad bin Amr bin Hazm, that he heard Urwa, and he related the hadith. Through Malik's route, Abu Dawud (181) and Nasai (163) also narrated it.In a version found in Nasai (1/101), Urwa said, "I kept disputing with Marwan until he summoned a man from his guard and sent him to Busra to ask her about what had been reported. Busra sent back the same account that Marwan had related to me from her."Urwa's hearing directly from Busra is confirmed as sahih in the hadith of Tirmidhi (1/126) and Ibn Majah (479), for both of them narrated it through another route from Hisham bin Urwa, who said, "My father told me, from Busra bint Safwan, that the Prophet ﷺ said, 'Whoever touches his penis should perform wudu.'" And in Tirmidhi's version, "He should not pray until he performs wudu."It appears that Urwa narrated it through two routes, once from Marwan from Busra, and once from Busra herself directly. It would be a mistake to treat this difference as grounds for weakening the hadith.For more, see Minna Kubra (1/43).