Musannaf Ibn Abi Shayba 1:863

Hammad bin Khalid narrated to us, from al-Umari, from Ubaydullah bin Umar, from al-Qasim, from Aisha, from the Prophet (peace be upon him), who said: "If one of you wakes up and sees wetness, but does not see that he had a wet dream, then he must perform ghusl (ritual bath). And if he sees that he had a wet dream, but does not see wetness, then there is no ghusl upon him."