What has to be done if one of the sajdahs is missed while performing a prayer, performing the prayer again or sajdah as-sahw?

Is sajdah as-sahw enough or is it necessary to perform the prayer again if a person forgets to perform one of the sajdahs in a rak'ah? Are two sajdahs or only one sajdah fard?  

Dear Brother / Sister,

If one of the sajdahs in a rak'ah is missed and if it is remembered at the end of the next rak'ah or the sitting, the sajdah is performed immediately and the prayer does not have to be performed again because of delaying it. 

If a person remembers it in the last sitting, he performs the missed sajdah first and performs the last sitting again. After that, he needs to perform sajdah as-sahw. Thus, the last rak'ah is performed with five sajdahs and three sittings. For, there are two sajdahs in a rak'ah.   

If a part of the prayer that is repeated is delayed to the end of the prayer, it is not regarded as abandoning fard; therefore, the prayer does not have to be performed again.

Two sajdahs are performed in each rak'ah. If one of them is missed deliberately, the prayer is invalidated. If a person misses a sajdah by mistake and remembers it after he finishes the prayer, he needs to perform this sajdah when he remembers it if he has not done anything contrary to the prayer; after that, he needs to perform the last sitting again and perform sajdah as-sahw.  

(see Ömer Nasuhi BİLMEN, Büyük İslam İlmihali)

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