Musnad Ahmad 5:471

Yahya ibn Sa'id narrated to us, from Hisham, my father informed me, that Ali said to al-Miqdad: "Ask the Messenger of Allah, may Allah bless him and grant him peace, about a man who draws near to his wife and emits madhy (pre-seminal fluid), for I am too shy to ask him because his daughter is with me." The Messenger of Allah, may Allah bless him and grant him peace, said: "He shall wash his penis and his testicles, and perform wudu."