Is ghusl obligatory for someone who had a wet dream and woke up but found no trace of semen, and there was some wetness in the anal area that he does not know if it is from istinja' (cleansing after defecation) or from the toilet, and there is no trace of semen or madhy (pre-seminal fluid) on it, knowing that he suffers from excessive doubt regarding ritual purity?

Ghusl is only obligatory upon the emission of semen. Therefore, if one has a wet dream but no semen is emitted, ghusl is not required. This is based on the Prophet's (peace and blessings be upon him) saying: "Water (ghusl) is only from water (semen)." And his saying to Umm Sulaym when she asked him about a woman seeing what a man sees: "Yes, if she sees water (semen)." Any wetness from which semen is not seen does not necessitate ghusl. We advise you to abandon and turn away from obsessive thoughts (waswasah).