[إذا استيقظ أحدكم من نومه فرأى بللا ولم ير أنه احتلم اغتسل وإذا رأى أنه قد احتلم ولم ير بللا فلا غسل عليه]
The Prophet (peace and blessings be upon him) said:
"When one of you wakes from his sleep and sees moisture but does not recall having had a wet dream, he should take a ritual bath (ghusl); and if he sees that he had a wet dream but does not see any moisture, then a ritual bath is not obligatory upon him."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 330M