Response: If one who is fasting has had a wet dream during the day in Ramadhaan, then his fast is sound and is not affected by the wet dream, because he had not done so intentionally. However, it is obligatory for him to perform the ritual purification (ghusl) so as to perform the prayer. To be prompt in performing the ritual purification is better, but not obligatory.
And Allaah knows best.
Shaykh Ibn Fowzaan
Fataawa Ramadhaan – Volume 2, Page 440, Fatwa No.365;
al-Muntaqaa min Fataawa ash-Shaykh Saalih ibn Fowzaan – Volume 3, Page 162