Is ghusl obligatory for one who has a wet dream but finds no wetness, only its trace? And if its trace has dried and he remembers the wet dream at the time of Fajr, should he perform ghusl, thereby missing congregational prayer, and is he sinful for missing it?

If a person has a wet dream and finds traces of semen, whether wet or dry, ghusl (ritual bath) becomes obligatory, because the determining factor is the presence of semen, not its state. And whoever finds traces of semen after sleeping must perform ghusl, even if they miss the congregational prayer or the time for prayer expires, according to the majority of scholars.