Sayyidah Aishah (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) was asked about a man who sees wetness after sleep but does not remember having a wet dream. He (peace and blessings be upon him) said: "He should perform ghusl. And if he sees that he had a wet dream but does not see any wetness, then there is no ghusl upon him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 790