If they follow him while believing that he is correct, then their prayer is valid.

If they follow him out of ignorance or forgetfulness, then their prayer is valid, due to their excuse, because they committed a prohibited act through ignorance or forgetfulness. The evidence for this is His saying, exalted is He:

“Our Lord, do not take us to task if we forget or make a mistake.” [al-Baqarah 2:286]

If they follow him while knowing that he has added an extra unit and that it is unlawful to follow him in this addition, then their prayer is invalid, because they deliberately added to the prayer.

If they separate from him, then their prayer is valid, because they have fulfilled the obligation required of them.

Question: Must the congregant alert his imam if he stands for an additional unit, or is this not obligatory?

Answer: He must alert him, due to the Prophet’s ﷺ statement: “If I forget, then remind me.” (1) A command indicates obligation.

If someone other than a congregant knows that a person praying has added an extra unit—for example, a man praying beside him who stands for a fifth unit, while he is not his imam—must he alert him?

Answer: The apparent view of the jurists’ statements is that he is not required to alert him if he is not his imam, because there is no connection between him and that person’s prayer. However, when we refer back to the general meaning of Allah’s saying:

“Cooperate with one another in righteousness and piety, and do not cooperate with one another in sin and transgression.” [al-Ma'idah 5:2]

we find that this falls under cooperation in righteousness. Thus, in my view, the correct position is that he must alert him—just as, if you saw someone intending to perform ablution with impure water, you would be obliged to alert him, even though there is no connection between you and him.