Musannaf Ibn Abi Shayba 1:987

Muhammad ibn Bishr narrated to us, he said: Isma'il ibn Abi Khalid narrated to us, from al-Harith ibn Shubayl, he said: Ali said: "I was a man who produced much madhy (prostatic fluid), so whenever I saw something of that, I would perform ghusl (full ritual bath). This reached the Prophet, peace and blessings of Allah be upon him, and he commanded me to perform wudu (ablution)."