Reference: Fataawa ash-Shaykh al’Allaamah Muhammad Naasiruddeen al-Albaanee fil-Madeenah wal-Imaaraat – Page 67
Question: What is the ruling regarding the one who wakes up from his sleep and finds dampness [on his clothing], yet he does not remember having had a wet dream?
Response: If the dampness which he finds is semen, then it is obligatory upon him to take a ritual bath [of purification], whether he remembers having had a wet dream or not.