It is narrated from Urwah (may Allah have mercy on him) that Sayyiduna Ali (may Allah be pleased with him) said: I said to Miqdad (may Allah be pleased with him): Ask the Messenger of Allah (peace and blessings be upon him), for were it not that his daughter is married to me, I would have asked him myself, about one of us who, when he draws near to his wife, emits madhi (pre-seminal fluid) which he cannot control, though he has not even touched her. So Sayyiduna Miqdad (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him), and the Messenger of Allah (peace and blessings be upon him) said: "When any of you emits madhi and has not touched her, let him wash his private organ and his testicles."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17697