Musnad Ahmad 5:671

Yazid narrated to us, Sharik informed us, from al-Rukin ibn al-Rabi’, from Husayn ibn Qabisah, from Ali, who said: I was a man who frequently produced madhy (prostatic fluid), and I was too shy to ask the Messenger of Allah (peace and blessings of Allah be upon him) because of his daughter. So, I ordered al-Miqdad to ask the Messenger of Allah (peace and blessings of Allah be upon him) about a man who finds madhy. He said: "That is the water of the stallion, and every stallion has water. So let him wash his penis and his testicles, and let him perform wudu as he performs it for prayer."