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