Musnad Ahmad 5:249

'Abdullah narrated to us, Muhammad ibn Ja'far al-Warkani narrated to me, Abu Shihab al-Hannat 'Abd Rabbih ibn Nafi' informed us, from al-Hajjaj ibn Artah, from Abu Ya'la, from Muhammad ibn al-Hanafiyyah, from 'Ali ibn Abi Talib, who said: When the matter of madhy (pre-seminal fluid) caused me difficulty, I ordered al-Miqdad to ask the Messenger of Allah, peace and blessings of Allah be upon him, about it. He said: "From it is the wudu (ablution)," out of shyness because of Fatimah.