What is the ruling on the prostration of forgetfulness (sujood as-sahw) for an Imam who said "Allahu Akbar" instead of "Sami' Allahu liman hamidah" when rising from rukoo' (bowing), then corrected himself, and what should a follower (ma'moom) do if he has not completed his tashahhud and is delayed in following the Imam in the prostration of forgetfulness?

The Imam's saying "Sami'a Allahu liman hamidah" (Allah hears the one who praises Him) is obligatory. If the Imam remedies its omission, it negates the obligation of Sujud As-Sahw (prostration of forgetfulness). However, it is prescribed for him to prostrate for uttering a prescribed saying in the wrong place. As for the congregant's prostration with the Imam, the Hanbali school holds that the congregant prostrates with him even if he has not completed the Tashahhud, and then completes it after prostrating. In contrast, the Shafi'i school believes it is obligatory for the congregant to delay prostrating to complete the Tashahhud first, because prostration is a補償 (jaber) and does not occur before the completion of an obligation. Therefore, whoever prostrates before completing it, their prayer is invalidated if they were aware and intentional. Based on this, if the questioner prostrated before completing the Tashahhud and did not utter it after prostrating, their prayer is invalid and they must repeat it.