Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.050.

Section : If one has a wet dream but does not find any traces of ejaculation.

There is no need for ghusl in this instance either. Ibn al-Munzhir said, “All of the knowledgeable people known to me agree on this point.” The hadith of Umm Salamah mentioned earlier supports this proposition.