Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Umm al-Mu'minin Sayyida Umm Salamah (may Allah be pleased with her) that the wife of Sayyiduna Abu Talhah al-Ansari (may Allah be pleased with him), Umm Sulaym (may Allah be pleased with her), came to the Messenger of Allah (peace and blessings be upon him) and said: "O Messenger of Allah! Indeed, Allah is not shy of (telling) the truth. If a woman experiences a wet dream, is ghusl (ritual bath) obligatory upon her?" The Prophet (peace and blessings be upon him) replied: "Yes, if she sees water."