Abd al-Rahman bin Namir said: I asked Imam al-Zuhri about a woman touching her private part: must she perform ablution? He said: Abdullah bin Abi Bakr narrated to me, from Urwah, from Marwan bin al-Hakam, the narration of Busrah bint Safwan (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "When anyone's hand touches his private part, let him perform ablution," and a woman will do the same.
(b) The apparent meaning of this hadith shows that "and a woman likewise" is the saying of al-Zuhri.
What also indicates this is that all the other narrators narrated it from al-Zuhri without this addition. It was also narrated in the hadith of Isma'il bin Ayyash from Hisham bin Urwah from his father, but it is not mahfuz (preserved).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 636