What should one do if he fasted relying on the sighting of the moon, and then it became clear to him at the end of Ramadan that he had fasted only twenty-eight days?

The questioner : The second section states: If a person fasted on Monday following the sighting of the crescent moon, how do we determine the day of the festival? Do we follow the sighting or the calculation? And if we have people who fasted on Monday and people who fasted on Tuesday, how is the day of the festival determined? Is it one day or two?

The Shaykh : The festival of 'Id al-Fitr is only one day. If these people fasted twenty-nine days, their fasting is complete, as mentioned previously in the hadith that the month may be twenty-nine days. Their fasting is complete, if Allah wills. However, if they fasted twenty-eight days, they must make up that one day after the festival. I say: The one who fasted twenty-nine days has fulfilled the month obligatory upon him. As for the one who fasted twenty-eight days, he must make up one day. As for the day of the festival, which is the festival of this fasting person who tied his fasting to the sighting of the crescent moon and not to the astronomical calculation, it is not permissible to fast on this day, due to what is established in many hadiths regarding the prohibition by the Prophet (peace and blessings be upon him) of fasting on the day of the festival.