Musnad Ahmad 1101:18

Yahya ibn Sa'id narrated to us, from Ibn Jurayj, 'Ata' narrated to us, from 'A'ish ibn Anas al-Bakri, who said: 'Ali, 'Ammar, and al-Miqdad discussed *madhy* (pre-seminal fluid). 'Ali said: "I am a man who produces much *madhy*, and I am shy to ask him (the Prophet) because of his daughter being with me (as my wife)." So he said to one of the two—either 'Ammar or al-Miqdad—('Ata' said: He named him to me, but I forgot), "Ask the Messenger of Allah." So he asked him, and he said: "That is *madhy*. One should wash that from himself." I said: "What is 'that from himself'?" He said: "His private part, and he should perform ablution, and he should perform his ablution well—or he should perform ablution like his ablution for prayer—and he should sprinkle water on his private part," or "his private parts."