Musnad Ahmad 5:284

Abu Ahmad narrated to us, Ruzam ibn Sa'id al-Taymi narrated to us, from Jawwab al-Taymi, from Yazid ibn Shurayk—meaning al-Taymi—from Ali, who said: I was a man who produced much prostatic fluid (madhy), so I asked the Prophet, peace and blessings of Allah be upon him. He said: "If you ejaculate (kadhfta), then perform ghusl for janabah, and if you are not ejaculating, then do not perform ghusl."