It is narrated from Muhammad ibn al-Hanafiyyah that Sayyiduna Ali (may Allah be pleased with him) said: I felt too shy to ask the Prophet (peace and blessings be upon him) about madhi (pre-seminal fluid), because his daughter was married to me. So I told Sayyiduna Miqdad (may Allah be pleased with him) to ask him (peace and blessings be upon him), and he (peace and blessings be upon him) said: "It requires ablution."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17695