Sayyiduna Urwah bin al-Zubayr (may Allah have mercy on him) said: During his governorship in Madinah, Marwan bin al-Hakam wrote 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 the private part. Marwan said: Sayyidah Busrah bint Safwan (may Allah be pleased with her) informed me that she heard from the Messenger of Allah (peace and blessings be upon him); it was mentioned before him (peace and blessings be upon 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 a guard was sent to Sayyidah Busrah (may Allah be pleased with her) to ask her what she narrated about this, and Sayyidah Busrah (may Allah be pleased with her) wrote back to them the same thing that Marwan had narrated to me.
This is the authentic version of al-Zuhri's hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 635