I asked the Messenger of Allah ﷺ about a woman who experiences a wet dream in her sleep. He said, "If she sees the fluid, let her perform the ritual bath."
أخبرنا يوسف بن سعيد قال حدثنا حجاج عن شعبة قال سمعت عطاء الخراساني عن سعيد بن المسيب
سألت رسول الله ﷺ عن المرأة تحتلم في منامها فقال إذا رأت الماء فلتغتسل
[Hasan] Ibn Majah related it in Kitab Tahara, Bab fi al-Mara Tara fi Manamiha ma Yara al-Rajul, no. 602, from the hadith of Saeed bin Musayab, transmitted through him, and it is also in al-Kubra, no. 204. This hadith has many corroborating narrations, including the preceding hadith.