Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 207
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.