Sahih Bukhari Volume 8, Book 73, Hadith Number 113.

Narated By Zainab bint Um Salama : Um Sulaim said, “O Allah’s Apostle! Verily Allah is not shy of (telling you) the truth. Is it essential for a woman to take a bath after she had a wet dream (nocturnal sexual discharge)?” He said, “Yes, if she notices discharge. On that Um Salama laughed and said, “Does a woman get a (nocturnal sexual) discharge?” He said, “How then does (her) son resemble her (his mother)?”