When does Sajda Sahw become Wajib?

Every mistake in Salaat does not make Sajda Sahw compulsory (Wajib).

 

What does not make Sajda Sahw Wajib:

 

Leaving any Masnoon (Sunnah) or Mustahab (Commendable) act does not make Sajda Sahw Wajib.

 

What makes Sajda Sahw Wajib:

 

The following make Sajda Sahw Wajib:

 
Missing out on any Wajib act: like reciting Surah Fatiha, or another Surah after it, Tashahud, etc.

 

Distorting any Wajib act, like reciting softly in a Salaat in which the recitation is to be in a loud voice, delaying an article of Salaat, like reciting more than the "At Tahiyyaat" in the first Qaida (sitting) of the Salaat, which in turn delays the 3^rd Raka'at.

 

Preponing any article of Salaat, like performing Sajda before Ruku, repetition of an article of Salaat like performing 3 Sajdah in one Raka'at, in all these cases Sajda Sahw is compulsory.

 

If a person in the Salaat stays quiet for time enough to complete an act of Salaat, then it is called as delaying an act or delaying of compulsory article.  The time equal to completing an act is mentioned as time enough for the recitation of three Tasbihaat (glorifications).  Therefore, if a person keeps quiet for a period equivalent enough for the recitation of three glorifications (i.e. Subhana rabbi a'ala or Subhana rabbi al azeem or the like), then he/she should make the Sajda Sahw.

 

As given in (Raddul Muhtaar, Vol. No. 1, Pg. No. 558)

 

What if more than 1 mistake in the Salaat?

 

If in a Salaat, more than 1 mistake, which make Sajda Sahw Wajib occur, then only one Sajda Sahw will be enough for all.

 

Method of performing Sajda Sahw:

 
Please keep in mind that as per the Hanafi school of Fiqh, the method of Sajda Sahw as described in the Hadith is that after reciting "At Tahiyyat," one Salaam should be done on the right side, then 2 Sajdah should be performed and then "At Tahiyyat," Durud Ibrahim and Dua Mathoora should be recited.  As given in (Fatawa A'alamgiri, Vol. 1, Pg No. 126 and Pg No. 130)