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." (FR)
English reference (FR): Vol. 1, Book 1, Hadith 612 (FR)
Arabic reference (FR): Book 1, Hadith 655 (FR)