Reference: Sharh ‘Umdatil-Fiqh – Tape No.4, Question No.36
Question: Sometimes, when I wake up from sleep, I notice a wet stain, but do not know whether it is prostatic fluid or semen?
Response: This is [what is known as] a wet dream, this is a wet dream. If you do not know [that you have had a wet dream], nor felt anything, but noticed the wet stain, then you are required to have a bath [or shower]. This is based upon the [meaning of the] statement of the Messenger of Allaah (sal-Allaahu ‘alayhi wa sallam):
…meaning, ghusl is always mandatory for nocturnal discharge – whether or not you felt anything.