Musnad Abu Ya'la al-Mawsili 3:54

Ubayd Allah narrated to us, Ayyub ibn Waqid al-Kufi narrated to us, Yazid ibn Abi Ziyad narrated to us, from Abd al-Rahman ibn Abi Layla, from Ali ibn Abi Talib, who said: I was a man who produced much madhy (pre-seminal fluid), so I ordered al-Miqdad ibn al-Aswad to ask the Messenger of Allah, peace and blessings of Allah be upon him, about madhy. He said: "From it is wudu (ablution), and from semen is ghusl (full ritual washing)."