Musnad Ahmad 5:464

'Abd al-Rahman narrated to us, Za'idah narrated to us, from al-Rukayn ibn al-Rabi', from Husayn ibn Qabisah, from 'Ali, who said: I was a man who frequently produced prostatic fluid (madhy), so I asked the Prophet, peace and blessings of Allah be upon him. He said: "When you see the prostatic fluid, perform ablution (wudu) and wash your private part. And when you see the gushing of water (semen), perform a full ritual bath (ghusl)." I mentioned this to Sufyan, and he said: I have heard it from al-Rukayn.