Sayyida Aisha (may Allah be pleased with her) narrates that it was asked from the Prophet (peace and blessings be upon him): A man finds traces of wetness on his clothes but does not remember having a wet dream (what should he do)? He said: He should perform ghusl (ritual bath). (It was asked:) A man remembers having a wet dream but does not find any traces of wetness on his clothes? He said: Ghusl is not obligatory upon him.
Hadith Referenceالمنتقى ابن الجارود / كتاب الطهارة / 89