Musnad Ahmad 424:1

Yazid narrated to us, he said: Muhammad ibn Ishaq informed us, from Hisham ibn Urwah, from his father, from al-Miqdad ibn al-Aswad, he said: Ali said to me: "Ask the Messenger of Allah, peace and blessings of Allah be upon him, about a man who plays with his wife, and *madhy* (pre-seminal fluid) comes out of him without the water of life (semen)?" He said: "He shall wash his private part and perform ablution, like his ablution for prayer."