It was narrated from 'Aishah that:
The Prophet said: "If anyone of wakes up and sees some wetness, but he does not think that he had an erotic dream, let him have a bath. But if he thinks that he had an erotic dream but he does not see any wetness, then he does not have to take a bath."
English reference: Vol. 1, Book 1, Hadith 612
Arabic reference: Book 1, Hadith 655