Musnad Ahmad 1156:107

'Abbad ibn 'Abbad al-Muhallabi narrated to us, from Hisham ibn 'Urwah, from his father, from Zaynab bint Umm Salamah, from her mother, Umm Salamah, that Umm Sulaym asked the Prophet, may Allah bless him and grant him peace, and 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."