Musnad Ahmad 5:293

Aswad ibn Amir narrated to us, Isra'il narrated to us, from Abu Ishaq, from Hani' ibn Hani', from Ali, who said: I was a man who produced much prostatic fluid (madhy). Whenever I produced it, I would perform ghusl (full ritual bath). I ordered Al-Miqdad to ask the Prophet, peace and blessings of Allah be upon him, about it. He laughed and said: "For it, there is wudu (ablution)."