Musnad Ahmad 5:57

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Mundhir Abu Ya'la, from Muhammad ibn al-Hanafiyyah, from Ali, who said: I was a man who produced much prostatic fluid (madhy), and I was too shy to ask the Prophet, peace and blessings of Allah be upon him, about prostatic fluid. He said: So I said to Al-Miqdad: Ask the Messenger of Allah, peace and blessings of Allah be upon him, about prostatic fluid for me. He said: So he asked him, and the Messenger of Allah, peace and blessings of Allah be upon him, said: "In it is wudu (ablution)."