It is narrated from Sayyidah Khawlah bint Hakim (may Allah be pleased with her) that she asked the Messenger of Allah (peace and blessings be upon him) for a ruling about a woman who sees in her sleep what a man sees: is a ritual bath (ghusl) obligatory upon her? He (peace and blessings be upon him) said: "Yes, when she sees water (sexual fluid)."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20981