Musnad Ahmad 1156:141

Waki’ narrated to us, Hisham and Ibn Numayr narrated to us, they said: Hisham ibn ‘Urwah informed us, from his father, from Zaynab bint Umm Salamah, from Umm Salamah, who said: Umm Sulaym came to the Prophet, may Allah bless him and grant him peace, and asked him about a woman who sees in her sleep what a man sees. He said: "If she sees the fluid, then she must perform ghusl (ritual bath)." I said: I said, "You have disgraced the women! Does a woman have a wet dream?" The Prophet, may Allah bless him and grant him peace, said: "May your right hand be covered in dust, for how else would her child resemble her?"