Musnad Ahmad 5:330

'Abdullah narrated to us, Shayban Abu Muhammad narrated to me, 'Abd al-'Aziz bin Muslim—meaning Abu Zayd al-Qasmali—narrated to us, Yazid bin Abi Ziyad narrated to us, from 'Abd al-Rahman bin Abi Layla, from 'Ali, who said: I was a man who produced much prostatic fluid (madhy), so I asked the Messenger of Allah (peace and blessings of Allah be upon him) about that. He said: "Regarding prostatic fluid (madhy), there is ablution (wudu), and regarding seminal fluid (maniy), there is full ritual washing (ghusl)."