Musannaf Ibn Abi Shayba 1:985

Abu Bakr narrated to us, he said: Husayn bin Ali narrated to us, from Za'idah, from al-Rukayn, from Husayn bin Qabisah al-Fazari, 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. I was too shy to ask him, so I ordered a man to ask him. He said: "When you see the prostatic fluid (madhy), perform ablution (wudu) and wash your private part. And when you see the prostatic fluid (wady), sprinkle water [on your garment] and perform the ritual bath (ghusl)."