It is narrated from Sayyiduna Rafi' bin Khadij (may Allah be pleased with him) that Sayyiduna Ali (may Allah be pleased with him) told Sayyiduna Ammar (may Allah be pleased with him) to ask the Prophet (peace and blessings be upon him) about madhy (pre-seminal fluid), and he (peace and blessings be upon him) said: "He should wash his private part and perform ablution."
Hadith Referenceالمعجم الكبير للطبراني / باب الراء / 4425