Sayyidah Zaynab (may Allah be pleased with her) narrates from her mother, Sayyidah Umm Salamah (may Allah be pleased with her), who said: Sayyidah Umm Sulaym (may Allah be pleased with her) came and said: O Messenger of Allah! Must a woman take a ritual bath (ghusl) when she has a wet dream? He (peace and blessings be upon him) said: "Yes; when she sees the fluid, she should take a ritual bath."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20292