Maliks Muwatta Book 2, Hadith Number 87.
Section : Ghusl of a Woman when She Experiences the Same as a Man in Her Sleep.
Yahya related to me from Malik from Hisham ibn Urwa from his father from Zaynab bint Abi Salama that Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, said, “Umm Salama, the wife of Abu Talha al-Ansari, came to the Messenger of Allah, may Allah bless him and grant him peace, and said, ‘Messenger of Allah! Allah is not ashamed of the truth-does a woman have to do ghusl if she has had an erotic dream?’ He said, ‘Yes, if she sees any liquid.'”