In all cases, if two trustworthy persons alert him, he must follow what they say, unless he is certain that he himself is correct. If he does not return while not certain that he is correct, his prayer is invalid, because he has deliberately omitted an obligation: when two trustworthy persons say “Subḥān Allah” to alert him, he is required to return.

The evidence for this is that when Dhū al-Yadayn reminded the Prophet ﷺ that he had prayed only two rak'ahs, he did not accept his statement until he asked the Companions: “Is what Dhū al-Yadayn says true?” They replied, “Yes.” (1)

If only one man says “Subḥān Allah” to alert him, he is not required to return. The evidence is that the Prophet ﷺ did not accept the statement of Dhū al-Yadayn. However, if he considers him most likely to be truthful, he should follow what he says, according to the view that it is permissible to act on what one considers more likely; and this is the correct view.

Question: Suppose one man says “Subḥān Allah” in a manner indicating that the imam has added something to the prayer, while another says “Subḥān Allah” in a manner indicating that he has not added anything. Which of the two should he follow?

The answer is: their statements cancel one another out. Thus, if one of them says “Subḥān Allah” when he stands up, then, when he prepares to sit down, the other says “Subḥān Allah,” two conflicting statements have reached him, and they cancel one another out: each statement negates the other. He should then refer back to his own judgment and proceed accordingly.

Note: The author stipulates, as a condition for the obligation to follow the statement of two trustworthy persons, that he not be certain that he himself is correct. If he is certain that he is correct, it is prohibited for him to follow what they say. That is, if they say, “Subḥān Allah,” but he is certain that he is correct