Musannaf Ibn Abi Shayba 1:967

Hushaym narrated to us, from Mansur, from al-Hasan, from Ali, who said: I used to experience *madhy* (prostatic fluid), so I ordered al-Miqdad to ask the Prophet, peace and blessings of Allah be upon him, about that, because his daughter was with me, and I was too shy to ask him. He said: "Every stallion emits *madhy*. If it is *maniy* (semen), then it requires *ghusl* (full ablution), and if it is *madhy*, then it requires *wudu* (ablution)."