Musnad Ahmad 1251:4

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, and Hajjaj said: Shu'bah narrated to me, he said: I heard 'Ata' al-Khurasani narrating from Sa'id ibn al-Musayyib, that Khawlah bint Hakim al-Sulamiyyah—and she is one of the maternal aunts of the Prophet, may Allah bless him and grant him peace—asked the Prophet, may Allah bless him and grant him peace, about a woman who has a wet dream. The Messenger of Allah, may Allah bless him and grant him peace, said: "She must perform ghusl (ritual bath)."