Musnad Ahmad 5:306

Ubaydah ibn Humayd narrated to us, Yazid ibn Abi Ziyad narrated to me, 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, or he was asked about that. He said: "For prostatic fluid (madhy), there is ablution (wudu), and for seminal fluid (maniy), there is a full ritual bath (ghusl)."