What is the ruling on one who masturbates during the daytime in Ramadan, thinking that it does not break the fast?

Masturbation is Islamically forbidden, and its prohibition intensifies in Ramadan, as it invalidates the fast and constitutes an act of defiance towards Allah. If a fasting person engages in it, unaware that it breaks the fast, yet knowing its general prohibition, he incurs sin and must make up the fast. This is especially true if he has been negligent in learning religious rulings. He must repent and make up the days on which he committed this act.