Musnad Ahmad 5:305

Ubaydah ibn Humayd al-Taymi Abu Abd al-Rahman narrated to us, Rukin narrated to me, from Husayn ibn Qabisah, from Ali ibn Abi Talib, who said: I was a man who frequently produced madhy (pre-seminal fluid), so I would perform ghusl (full ritual bath) in the winter until my back cracked. He said: So I mentioned that to the Prophet, peace and blessings of Allah be upon him, or it was mentioned to him. He said: "Do not do that. When you see the madhy, wash your private part and perform wudu (ablution) as you do for prayer. But when you emit the fluid (semen) with force, then perform ghusl."