Musnad Ahmad 5:327

Abdullah narrated to us, Ishaq bin Isma'il narrated to us, Muhammad bin Fudayl narrated to us, from Yazid bin Abi Ziyad, from Abd al-Rahman bin Abi Layla, who said: I heard Ali saying: 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: "For it, there is ablution (wudu)."