What should a Muslim do if the crescent moon is sighted in some Islamic countries, while the country in which he resides has completed thirty days of Sha‘ban and Ramadan? And what is the reason for people's اختلاف (disagreement) in determining the beginning and end of Ramadan?

You should fast and break your fast with the people of your country, for the Prophet, peace and blessings be upon him, said: "The fast is the day you fast, and the breaking of the fast is the day you break your fast, and the sacrifice is the day you offer sacrifice." Disagreement is evil. The cause of disagreement is either seeing the crescent moon or not, or not trusting those who claim to have seen it. It is obligatory upon Muslims to fast and break their fast based on the sighting of the crescent moon. However, if people disagree, then fast and break your fast with the people of your country. It has been authentically reported that Ibn Abbas did not act upon the sighting of the people of Sham due to the difference in horizons.