Musannaf Ibn Abi Shayba 1:849

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, from Ikrimah, from Ibn Abbas, who said: If he has a wet dream but does not see any moisture, then there is no ghusl (ritual bath) upon him. And if he sees moisture but does not recall having a wet dream, then the ghusl is upon him.