Musnad Ahmad 5:307

Ubaydah narrated to us, Sulayman al-A’mash narrated to me, from Habib ibn Abi Thabit, from Sa’id ibn Jubayr, from Ibn Abbas, who said: Ali said: I was a man who produced much prostatic fluid (madhy), so I ordered a man to ask the Prophet, peace and blessings of Allah be upon him, about it. He said: "In it is wudu (ablution)."