Is it permissible to complete a compensatory fast after drinking water, thinking that the time for Fajr has not yet begun, or is it obligatory to break the fast and re-fast the day?

If a fasting person drinks after the dawn breaks, their fast becomes invalid. Scholars have differed regarding the obligation to complete the fast and make up for it (qada'):

Malikis and Hanafis: It is obligatory to complete the fast if one has begun it, and it is obligatory to make it up if it becomes invalid. However, Malikis do not consider making it up obligatory unless the fasting person intentionally invalidated their fast. Shafi'is and Hanbalis: It is recommended to complete the fast but not obligatory, and it is recommended to make it up if it becomes invalid. They cited the hadith: "The voluntary fasting person is the master of himself; if he wishes, he fasts, and if he wishes, he breaks his fast."

It is preferable to complete the fast to avoid the disagreement of those who consider it obligatory. Making it up is not obligatory according to the majority of scholars, even though making it up is preferable for those who hold it to be obligatory.