Musnad Ahmad 1156:33

Yahya ibn Sa'id narrated to us, from Hisham, who said: My father informed me, from Zaynab bint Umm Salamah, from Umm Salamah, who said: Umm Sulaym said: "O Messenger of Allah, indeed Allah is not ashamed of the truth. Is there a ghusl (ritual bath) upon a woman if she has a wet dream?" He said: "Yes, if she sees the water." Umm Salamah laughed and said: "Does a woman have a wet dream?" The Prophet, peace and blessings of Allah be upon him, said: "Then by what does the child resemble [the mother]?"