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Other example, if someone forgets the second sujoud in the first rak'a and he lifted for the second rak'a, he lowers down and makes the second sujoud then he completes his prayer and make Tasleem, then Sujuoud Al Sahw (prostrations of forgetfulness), then another Tasleem.

FORGETTING AL WAJIBAAT
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If anyone leaves one of the Wajibaat delibrately, then his prayer is invalid. If he remembers it immidiately, he performs it, and if he remembers before he reaches the other rukn, he returns and performs it, then he completes his prayer and makes Tasleem and then sujuoud al sahw. If he remembers it after reaching the next rukn he completes his prayer and performs sujoud al sahw before Al Tasleem.

EXAMPLE
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Someone while lifting to the third rak'a he remembers that he forgot the first Tashahud , so he sits down and makes the Tashahud, and completes his prayer without making sujoud al sahw.. If he remembers before standing, he sits down and makes Tashahud, completes his prayer, then makes Tasleem, then makes sujoud al sahw....If he stood up, he completes his prayer, makes sujoud al sahw after Al Tasleem.

النبيَّ صَلَّى اللهُ عليه وسلَّمَ صَلَّى بهِمُ الظَّهْرَ، فَقَامَ في الرَّكْعَتَيْنِ الأُولَيَيْنِ لَمْ يَجْلِسْ، فَقَامَ النَّاسُ معهُ حتَّى إذَا قَضَى الصَّلَاةَ وانْتَظَرَ النَّاسُ تَسْلِيمَهُ كَبَّرَ وهو جَالِسٌ، فَسَجَدَ سَجْدَتَيْنِ قَبْلَ أنْ يُسَلِّمَ، ثُمَّ سَلَّمَ.

The prophet, peace be upon him, stood up for the Zuhr prayer and he should have sat (after the second raka but he stood up for the third raka without sitting for Tashahud) and when he finished the prayer he performed two prostrations and said Takbeer on each prostration while sitting, before ending (the prayer) with Tasleem; and the people too performed the two prostrations with him instead of the sitting he forgot.” Sahih Bukhari.


This Hadith shows us:

It is obligatory to perform Sujuoud Al Sahw if anyone leaves the first Tashahud.

The first Tashahud is not rukn(pillar), as if anyone forgets any rukn, he should make this rukn, then makes sujoud al sahw...

One sujoud al sahw is sufficient for more than one
forgetfulness. As in the last example; the prophet forgot sitting down and Al Tashahud.

If the Imam forgets anything and performed sujoud al sahw the people who follow him should perform also sujoud al sahw like him, though he was the one who forgot .

It is important to follow the Imam, as the prophet, peace be upon him, consented them when they followed him and left sujuoud like him