What is the ruling on someone who ate, thinking that the Fajr adhan had not yet been called, then stopped eating upon hearing the adhan, whether it was for an obligatory fast like Ramadan, a compensatory fast, an oath, or other types of obligatory fasts, or for a supererogatory fast?

If someone ate, thinking that Fajr had not yet dawned, while observing an obligatory fast like Ramadan (definition), and it later became clear that they ate after Fajr had dawned, then they are required to abstain [from eating and drinking for the rest of the day] and make up that fast, according to the majority of scholars. However, if their fast was a supererogatory one, then they are not required to abstain nor to make up the fast. Some jurists, such as Sheikh Ibn Uthaymeen, hold the view that an obligatory fast does not need to be made up, although abstention for the rest of the day is still required. Nevertheless, the first opinion is more cautious and more absolving of responsibility.