Marwan bin al-Hakam said during his rule that ablution should be performed after touching the private part, when one's hand touches it. I objected to this and said: There is no ablution upon one who touches it. Marwan said: Busrah bint Safwan (may Allah be pleased with her) informed me that she heard the Messenger of Allah (peace and blessings be upon him); it was being mentioned before him what ablution is to be performed for, and the Messenger of Allah (peace and blessings be upon him) said: "Ablution is to be performed after touching the private part." Urwah (may Allah have mercy on him) said: I kept arguing with Marwan until Marwan sent one of his guards to Busrah (may Allah be pleased with her) to ask her what she said about this. Busrah (may Allah be pleased with her) sent back to him the same message that Marwan had narrated to me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 619