Musnad Ahmad 5:462

'Abd al-Rahman narrated to us, from Za'idah ibn Qudamah, from Abu Hasin al-Asadi; and Ibn Abi Bukayr narrated to us, Za'idah narrated to us, Abu Hasin al-Asadi informed us, from Abu 'Abd al-Rahman, from 'Ali, who said: I was a man who produced much prostatic fluid (madhy), and the daughter of the Messenger of Allah (peace and blessings of Allah be upon him) was my wife. So I ordered a man to ask him, and he said: "Perform ablution (wudu) and wash it."