What is the religious ruling for someone who fasted one day before the Muslims, assuming that Sha'ban had twenty-nine days, a practice widespread among some people in Najran?

It is not permissible to fast one or two days before Ramadan for precautionary reasons. The claim that Sha'ban does not exceed twenty-nine days is false. It has been authentically reported from the Prophet, peace and blessings be upon him, that the month can be twenty-nine nights, and he commanded to complete the count to thirty if the crescent moon is obscured.