Musnad Ahmad 1153:2176

Hammad ibn Khalid narrated to us, from Abdullah, from his brother Ubaydullah, from al-Qasim, from Aisha, who said: The Messenger of Allah, peace and blessings of Allah be upon him, was asked about a man who finds wetness but does not remember having a wet dream. He said: "He must perform ghusl (ritual bath)." And [he was asked] about a man who thinks he has had a wet dream, but does not see any wetness. He said: "There is no ghusl upon him." Umm Sulaym then said: "Is there anything upon a woman if she sees that?" He said: "Yes, for women are the counterparts of men."