Under what circumstances must the Imam accept the notification of a single follower?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
If only one follower alerts the Imam, the Imam is not obligated to accept his statement unless that one person's veracity strongly outweighs his own perception of correctness. The Prophet (peace be upon him) did not accept the statement of Dhu al-Yadayn when he was alone in his notification. If the one notifier is an openly immoral person (fasīq), his statement is not accepted because the testimony of such a person is not considered in matters of religious rulings.
Supporting text
If the followers divide into two groups, one confirming the error and another opposing it, their testimony nullifies each other due to mutual contradiction, similar to two conflicting pieces of evidence. If the Imam does not correct himself, and a follower is certain the Imam is in error, the follower should not follow the Imam in subsequent actions of the prayer, though this specific issue (the deviation itself) is not part of the prayer actions requiring immediate abandonment. In this case, the follower should wait for the Imam to conclude, as the Imam's prayer remains valid concerning the excess unless he fails to compensate for it.