It is narrated from Sayyidah Khawlah bint Hakim (may Allah be pleased with her) that she asked the Prophet (peace and blessings be upon him): O Messenger of Allah, a woman sees in her sleep what a man sees (that is, a wet dream). He (peace and blessings be upon him) said: "When she sees that, let her take a ritual bath (ghusl)."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20980