What is the ruling on an Imam's oversight in the third Rak'ah where he sat, and being alerted by saying "SubhanAllah", then his oversight in the final Tashahhud regarding the prostration of forgetfulness (sujud al-sahw) and his saying one Taslimah, and being alerted before the second Taslimah to perform the prostration of forgetfulness and then say Taslim? Is this action correct, or is something else required of them?

If someone adds to their prayer out of forgetfulness, such as the imam sitting mistakenly, they must perform the prostration of forgetfulness (sujood as-sahw), and this prostration should be done after the salam, based on the hadith of Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace be upon him) prostrated two prostrations after he had given the salam when he added to his prayer.

Scholars have differed on the timing of the prostration of forgetfulness, whether it is before or after the salam, and the difference is regarding what is preferable. However, if one prostrates for forgetfulness either before or after the salam, their prayer is valid, and there is nothing against them.

Imam An-Nawawi (may Allah have mercy on him) said: "There is no dispute among these differing scholars and others that if one prostrates before the salam or after it for an addition or omission, it is sufficient, and their prayer is not invalidated. Their difference is only regarding what is more preferable."

Therefore, the imam should have been given time and not alerted, as he might have intended to prostrate after the salam. However, the alerting by the worshippers made him flustered and return to prostrating before the salam. His prayer is valid in any case, and so is the prayer of those behind him, and there is nothing against them.