Musnad Ahmad 5:45

'Abdullah narrated to us, Muhammad ibn 'Abdullah ibn Numayr narrated to me, Waki' narrated to us, al-A'mash narrated to us, from al-Mundhir, from Muhammad ibn 'Ali, from 'Ali, who said: I was a man who produced much *madhy* (prostatic fluid), and I was too shy to ask the Messenger of Allah, peace and blessings of Allah be upon him, because of his daughter. So I ordered al-Miqdad to ask him. He said: "He should wash his penis and perform ablution."