[The ruling concerning common believers in their fasting, breaking the fast, and standing at ʿArafah, and how they are to be guided]

Answer—and Allah is the One who grants success: The jurists of the madhhab have said: the madhhab of the layperson is the madhhab of whichever mujtahid he has agreed with, if he entered the matter believing in its correctness.
On that basis, some mujtahids held that: the fast is the day on which the people fast; the breaking of the fast is the day on which the people break the fast; and ʿArafah is the day on which the people stand at ʿArafah. They adduced a ḥadīth narrated to that effect. Thus the layperson who fasts, breaks the fast, and stands with the people—without investigation or scrutiny—has agreed with the view of some mujtahids; what he has done is therefore judged valid and sufficient.
He is not required thereafter to look into whether he was right or not; there is no basis for obligating him to investigate after we have already ruled the act valid—except that, if it becomes certain to him that he erred in fasting or breaking the fast, then he must act in accordance with what has become certain to him. As for the standing at ʿArafah, that cannot be corrected except in some cases—namely, if it becomes certain to him that he stood on the eighth day while there still remains some of the ninth day in which he can stand; then it is obligatory for him to stand.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1