Sayyidah Umm Salamah (may Allah be pleased with her) says that Umm Sulaym (may Allah be pleased with her), the wife of Abu Talhah al-Ansari (may Allah be pleased with him), came to the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, indeed Allah Almighty is not shy of stating the truth; is ghusl required of a woman when she has a wet dream? He (peace and blessings be upon him) said: "Yes, when she sees the water."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 791