Musnad Abu Ya'la al-Mawsili 209:17

Muhammad ibn Abd Allah ibn Numayr narrated to us, my father narrated to us, Hisham narrated to us, from his father, from Zaynab bint Umm Salamah, who said: Umm Sulaym entered upon the Messenger of Allah (may Allah bless him and grant him peace) and said: "Indeed, Allah is not shy of the truth. Is there a ritual bath (ghusl) upon a woman if she has a wet dream?" He said: "Yes, if she sees the water." Umm Salamah said: "And does a woman have a wet dream?" He said: "May your right hand be covered in dust (a term of endearment/exclamation), then from what does her child resemble her?"