Musnad Ahmad 1251:3

Waki’ narrated to us, from Sufyan, from Ali ibn Zayd ibn Jud’an, from Sa’id ibn al-Musayyib, from Khawlah bint Hakim, that she asked the Prophet, peace and blessings of Allah be upon him, about a woman who sees in her sleep what a man sees. He said: "There is no ghusl (ritual bath) upon her until she discharges the fluid, just as there is no ghusl upon a man until he discharges."