Musnad Ahmad 5:328

Abdullah narrated to us, Wahb ibn Baqiyyah al-Wasiti narrated to me, Khalid informed us, from Yazid ibn Abi Ziyad, from Abd al-Rahman ibn Abi Layla, from Ali, who said: I was a man who produced much prostatic fluid (madhy), so I asked the Prophet, peace and blessings of Allah be upon him. He said: "In it is wudu (ablution), and in the case of semen (mani) is ghusl (full ritual washing)."