Musnad Abu Ya'la al-Mawsili 3:102

Abu Khaythama narrated to us, Humayd ibn Abd al-Rahman narrated to us, Hasan narrated to us, from Bayan, from Husayn ibn Safwan, from Ali, who said: I was a youth who produced much prostatic fluid (madhy). When the Messenger of Allah (peace and blessings of Allah be upon him) saw that the fluid was troubling me, he said: "Ghusl (full ablution) is only required from the spurting fluid (mani)."