Sahih Muslim Book 3, Hadith Number 610.

Chapter : Bathing is obligatory for a woman after experiencing orgasm in dream.

Umm Salama reported: Umm Sulaim went to the Apostle of Allah (may peace be upon him) and said: Apostle of Allah, Allah is not ashamed of the truth. Is bathing necessary for a woman when she has a sexual dream? Upon this the Messenger of Allah (may peace be upon him) said: Yes, when she sees the liquid (vaginal secretion). Umm Salama said: Messenger of Allah, does a woman have sexual dream? He (the Holy Prophet) said: Let your hand be covered with dust, in what way does her child resemble her?