It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he said: I was a man who had a great deal of madhi (pre-seminal fluid), and my wife was the daughter of the Prophet (peace and blessings be upon him), (so) I felt shy to ask him (peace and blessings be upon him) (about it). I told a man, and he asked him (peace and blessings be upon him) (about madhi). He (peace and blessings be upon him) said: "When you find that, wash your private part and perform ablution."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1665