Musnad Abu Ya'la al-Mawsili 3:197

Abu Khaythama narrated to us, Jarir narrated to us, from Yazid ibn Abi Ziyad, from Abd al-Rahman ibn Abi Layla, from Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked about prostatic fluid (madhy), and he said: "Regarding it is ablution (wudu), and he should wash it." And regarding semen (maniy), there is the ritual bath (ghusl).