This ruling is of the same type as I have mentioned before: sometimes Allah the Exalted makes a command in His Book conditional and obligatory, then makes the same command unconditionally obligatory through the tongue of the Prophet (peace be upon him). For Allah the Exalted did not mention madhiy (pre-seminal fluid) in the verse of wudu, whereas the Noble Prophet (peace be upon him) made wudu obligatory due to madhiy. All the early and later scholars of all cities are unanimous that wudu becomes obligatory due to madhiy.

It is narrated from Sayyiduna Ali bin Abu Talib (may Allah be pleased with him), he says: I was a person who had a lot of madhi (pre-seminal fluid), and I felt shy to ask the Messenger of Allah (peace and blessings be upon him) about this because his daughter was married to me. So I instructed a man to ask (this issue), and he asked the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) said: "Wudu must be performed due to the discharge of madhi."