Shaykh al-Islām—may Allah have mercy on him—was asked:

Some inhabitants of a city sighted the crescent of Dhū al-Hijjah, but the sighting was not established before the city’s judge. May they fast on the day outwardly reckoned as the ninth, even if it is actually the tenth?

He replied:

Yes, they should fast on the day outwardly recognized by the community as the ninth, even if it is in fact the tenth, and even supposing that sighting were established.

In the Sunan collections, Abū Hurayrah reports that the Prophet, peace and blessings be upon him, said:

“Your fasting is on the day you all fast, your breaking of the fast is on the day you all break the fast, and your sacrifice is on the day you all offer sacrifices.”

It was recorded by Abū Dāwūd, Ibn Mājah and al-Tirmidhī, who graded it authentic. ‘Ā’ishah—may Allah be pleased with her—also reported that the Messenger of Allah, peace and blessings be upon him, said:

“The day of breaking the fast is the day people break their fast, and the day of sacrifice is the day people offer their sacrifices.”

This was narrated by al-Tirmidhī, and this is the practice followed by all the imams of the Muslims. For if people mistakenly stood at ‘Arafah on the tenth day, their standing there would be valid