Sahih Bukhari Volume 001, Book 003, Hadith Number 132.
Narated By Um Salama : Um-Sulaim came to Allah's Apostle and said, "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?) The Prophet replied, "Yes, if she notices a discharge." Um Salama, then covered her face and asked, "O Allah's Apostle! Does a woman get a discharge?" He replied, "Yes, let your right hand be in dust (An Arabic expression you say to a person when you contradict his statement meaning "you will not achieve goodness"), and that is why the son resembles his mother."