Sahih Bukhari Volume 001, Book 005, Hadith Number 280.
Narated By Um-Salama : (The mother of the believers) Um Sulaim, the wife of Abu Talha, came to Allah's Apostle and said, "O Allah's Apostle! Verily Allah is not shy of (telling you) the truth. Is it necessary for a woman to take a bath after she has a wet dream (nocturnal sexual discharge)?" Allah's Apostle replied, "Yes, if she notices a discharge."