Musnad Ahmad 5:445

Waki' narrated to us, Hisham bin 'Urwah narrated to us, from his father, who said: Ali said: I was a man who produced much prostatic fluid (madhy), and I was too shy to ask the Prophet, peace and blessings of Allah be upon him, because of his daughter's position [as my wife]. So I ordered Al-Miqdad, and he asked him. He said: "He shall wash his penis and his testicles, and perform wudu."