Experiencing a wet dream while sleeping does not break the fast, as it is something that occurs beyond one’s control. However, one must perform ghusl (ritual purification) before the time for prayer passes.
ʿĀ’ishah and Umm Salamah (may Allah be pleased with them both) narrated:
“The Messenger of Allah (peace and blessings be upon him) would sometimes wake up in the morning in a state of major ritual impurity during Ramadan, then perform ghusl and continue fasting.”
(Reported by al-Bukhārī and Muslim)