Issue: Suppose the imam does not hold that the prostration of forgetfulness is obligatory, while the follower holds that it is obligatory. For example, regarding the first tashahhud: some scholars, in accordance with the Shafi‘i school, consider it a sunnah rather than an obligation. Thus, if the imam omits it and does not perform the prostration of forgetfulness, on the basis that it is a sunnah and that omitting a sunnah does not require the prostration of forgetfulness, must the follower—who holds that the prostration of forgetfulness is obligatory—perform it?

The answer is no, because his imam holds that no prostration is due from him, and the follower’s prayer is connected to the imam’s prayer. No deficiency has occurred on the follower’s part; rather, he is required to follow the imam, and he has done what is required of him.

However, if the imam holds that the prostration of forgetfulness is obligatory and says “Subhan Allah” to indicate that he should prostrate, but then does not prostrate, the jurists ﵏ (1) said: The follower should prostrate once he has lost hope that his imam will prostrate. This is because his prayer is connected to the imam’s prayer, and the imam has done something that necessitates prostration but omitted the prostration without any interpretive basis. The follower must therefore make up for this deficiency by prostrating.

The prostration of forgetfulness is obligatory for that which, if done deliberately, invalidates the prayer …

His statement, “The prostration of forgetfulness is obligatory for that which, if done deliberately, invalidates the prayer”: this is the criterion for matters that require the prostration of forgetfulness. Thus, the prostration of forgetfulness is obligatory for every matter whose deliberate omission invalidates the prayer.

For example, if you omit saying, “My Lord, forgive me,” between the two prostrations, the prostration of forgetfulness is obligatory upon you, because if you were to omit it deliberately, your prayer would be invalid.