From Sahih Bukhari (Hadith 6121):
Narrated Um Salama:Um Sulaim came to Allah's Messenger (ﷺ) and said, "O Allah's Messenger (ﷺ)! 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
Reference: Bukhari 6121