Musannaf Ibn Abi Shayba 1:966

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Yazid ibn Abi Ziyad, he said: Abd al-Rahman ibn Abi Layla narrated to me, from Ali, he said: The Prophet, peace and blessings of Allah be upon him, was asked about prostatic fluid (madhy), and he said: "Regarding it is ablution (wudu)." And regarding semen (maniy), there is the ritual bath (ghusl).