Sahih Bukhari | Volume 8 | Hadith Number 142

Narrated by Um Salama
Um Sulaim came to Allah's Apostle and said, "O Allah's Apostle! Verily, Allah does not feel shy to tell the truth. If a woman gets a nocturnal sexual discharge (has a wet dream), is it essential for her to take a bath? He replied, "Yes if she notices a discharge."