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What should a person do if the imam forgets a sujood in prayer and the mamoom (the one(s) who prayers behind the imam, as following him) indicates, but the imam continues and does not do Sajdatu Sahu.
Continue praying with the imam
Stop praying with the imam.
Do Sajdatu Sahu after the prayer separately?
Forgetting only one Sujood (prostration) does not make prayer void. In this case the Imam Should perform two Sajda Sahw after prayer and person behind him can continue the Jamaat prayer properly behind him.
Sajda Sahw should be done after end of prayer. And not inside the prayer.
The duty of Imam of prayer and who is praying behind Imam of prayer are different.
Generally whenever the person behind Imam of prayer see the prayer of Imam became void then his duty is to change his intention from Jamaat prayer into individual prayer and continue his prayer in individual form. Anyway if the prayer of Imam became void the person behind him is not permitted to break his own prayer. If the person dislike to leave the Jamaat prayer another possible method is that the person select another person who is praying in front of him as Imam of prayer (in his mind) and continue Jamaat prayer behind him as Imam.
References:
The ma’mumin (those following the Imam in Salat) should inform the Imam, they should do this with reciting zekrs aloud. If Imam doesn't pay attention you should change your intention from Jamaat to individual prayer and do sajdah (prostration) and continue the right Salat.
pay attention Imam can do sajdah sahw after salat and his salat will be right (because he didn't understand that one sajdah where to stay and It was sahwy) but if ma’mumin set off that one sajdah where to stay could not do sajdah sahw or continuing Salat with Imam;they should change intention.(if he/she continue with Imam,his/her Salat will be void.)
Reference (in Persian):
The answers given so far are reflecting the shi'a perspective. As the question was more general, please find below the sunni perspective.
If the Imam didn't react on the invocation of "subhan Allah" the ma'mums should follow the Imam until the end because of this hadith or this. Once he finished they should tell him and then they will have to do sujud as-sahw. This applies if the Imam didn't forget something which is considered as fard/farz.
And here is a reference and an other Version of the hadith.
But if he forgot a farz/fard the matter would be a bit more difficult (because of different opinions and interpretations of the hadith): It depends on: did the ma'mum know whether this was a mistake of the Imam or not, if no if he followed the Imam his prayer would be valid. If he knew then some scholars say one should complete the prayer in a valid matter as an example they said if an Imam stood up for a 5th rak'a if a ma'mum said "subhan Allah" and the Imam continued this ma'mum should finish his prayer, even if the others followed the Imam, but if two or more ma'mums tried to inform him then if they were right all ma'mums should end the prayer with the salam and leave the Imam pray the 5th rak'a. For the Imam himself if he was certain that his prayer was sane then he only has to do sahw, if not and he didn't react his prayer is invalid and he will have to repeat it. See here for example 658 or here (both in Arabic) and here in English.
You never do sujud as-sahw during the prayer but at the end of a prayer, this means before or after salam! Of course unless you have joined a prayer later then you might do it during the prayer and after finishing again, as the sujud you did was an addition to your prayer!
Some times the invocation of ma'mums can even prevent the sujud, for example if the Imam wanted to stand up and missed a sajda but was "informed" before his hands left the ground and react the good way!
According to at-Tirmdihi in a (not translated) commentary of the Hadith (of Dhul-Yadain) narrated by abu Huraira some people of Kufa said if someone speaks during his prayer because he forgot or not knowing that this is forbidden he should repeat his prayer. Imam ash-Shafi'i and Malik accepted the hadith as is, which means that in such a case one could speak. While Imam Ahmad said if the Imam was thinking his prayer is complete then he could complete it as usual after being informed but the ma'mum who knew it wasn't complete should repeat it, as he broke his prayer by talking!
And Allah knows best!
