It is narrated from Umm al-Mu'mineen Umm Salamah (may Allah be pleased with her) that Umm Sulaym (may Allah be pleased with her), the wife of Abu Talhah, came to the Prophet (ﷺ) and said: "O Messenger of Allah (ﷺ)! Allah is not shy of the truth. Is it obligatory for a woman to perform ghusl when she has a wet dream?" He replied: "Yes, when she sees the fluid."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 93
Hadith Takhrijاخرجہ البخاری، العلم، باب الحياء فى العلم (130) (282)، ومسلم، الحيض، باب وجوب الغسل على مرأة بخروج المني منها (313).