Musnad Ahmad 27:1186

Abu Abd al-Rahman narrated to us, Ibn Umar—meaning Abd al-Jabbar al-Ayli—narrated to us, Yazid ibn Abi Sumayyah narrated to us, I heard Ibn Umar saying: Umm Sulaym, who is the mother of Anas ibn Malik, asked the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, a woman sees in a dream what a man sees." The Messenger of Allah, may Allah bless him and grant him peace, said to her: "If a woman sees that and discharges, then she must perform ghusl (ritual bath)."