It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her); she said: Sayyidah Umm Sulaym (may Allah be pleased with her) came to the Messenger of Allah (peace and blessings be upon him) and said: "O Messenger of Allah, is it necessary for a woman to take a ritual bath (ghusl) when she has a wet dream?" He (peace and blessings be upon him) said: "Yes, when she sees a trace of the fluid."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20105