Abu Dawud Book 1, Hadith Number 207.
Chapter : On prostatic fluid madhi.
Narated By Al-Miqdad ibn al-Aswad : Ali ibn Abu Talib commanded him to ask the Apostle of Allah (PBUH) what a man should do when he wants to have intercourse with his wife and the prostatic fluid comes out (at this moment). (He said): I am ashamed of consulting him because of the position of his daughter. Al-Miqdad said: I asked the Apostle of Allah (PBUH) about it. He said: When any of you finds, he should wash his private part, and perform ablution as he does for prayer.