Musannaf Ibn Abi Shayba 1:858

Waki’ narrated to us, from Shu’bah, who said: I asked al-Hakam and Hammad about a man who wakes up and finds wetness. Al-Hakam said: He does not perform ghusl (ritual bath). And Hammad said: If he thinks that he has had a wet dream, he performs ghusl.