How to Perform Sajdah Sahv
After completing tashahhud in the final sitting position one turns one's face only to the right with salutation and then saying takbeer, performs two sajdahs, one after the other, peacefully. After the second sajdah, one has to repeat tashahhud, durood, etc and complete the prayer in usual way.
When Sajdah Sahv will Not Make the Prayer Valid
If a fard part of the prayer is ignored or missed intentionally or unintentionally.
If a wajib part is missed intentionally.
When Sajdah Sahv is Obligatory
If a person omits a wajib part of salat.
If a person causes undue delay in performance of a wajib or fard part of salat.
If a person repeats or overdoes something.
If a person recites the Qur'an audibly where it should be recited inaudibly or vice-versa.
If a person forgets to stand in qauma or to sit in jalsah.
If a person forgets to observe the first sitting (qa'dah ula) and stands up, he should not resume the sitting position on remembering the mistake, but should complete the prayer and perform sajdah sahv. However, if he realizes his mistake before standing up fully, he should sit down and need not perform sajdah sahv.
If a person forgets to observe final sitting position in a fard prayer containing two or four rak'ahs and stands up, he should sit down if he realizes the mistake before performing the sajdah (of the extra rak'ah) and carry out sajdah sahv to complete the prayer. If however he realizes his mistake after performing the sajdah (of the extra rak'ah), he should not sit but perform another rak'ah to complete four or six rak'ahs. He need not perform sajdah sahv, for this prayer will be regarded as nafl and fard prayer will have to be offered again.
If a person forgets to recite Al-Fatiha, Qunut, or tashahhud or the additional takbirs in Eid Prayer.
If the Imam omits or ignores wajib of the prayer, the follower has to perform sajdah sahv with the Imam. But if the follower omits a wajib neither of them has to perform sajdah sahv.
If a person omits to recite the additional surah in one or both of the first rak'ahs of the fard prayer he may recite the additional surah in one or both of the last rak'ah and perform sajdah sahv at the end.
If a person omits to recite the additional surah in any rak'ah of the sunnah or nafl prayer, he must perform sajdah sahv at the end.
If in the final sitting of a fard prayer or four rak'ahs a person has completed tashahhud and then has a doubt that this is his first sitting, he should stand up for the fifth rak'ah. Now if before performing the sajdah he realizes his mistake he should resume the sitting position and complete his prayer with sajdah sahv. But if he has performed the sajdah of the fifth rak'ah, he should add the sixth rak'ah as well and complete his prayer with sajdah sahv. The first four rak'ahs will be regarded as the fard prayer and the last two as nafl.
If the Imam or individual omits to recite Al-Fatiha in one or both of the last two rak'ahs of a fard prayer of four rak'ahs, he need not carry out sajdah sahv. But if he omits to recite Al-Fatiha in any of the sunnah or nafl rak'ahs, he will have to perform sajdah sahv because it is essential to recite Al-Fatiha in the first two rak'ahs of the fard prayer but in all the rak'ah of sunnah and nafl.
If a person carries out two rukus or three sajdahs in the same rak'ah or recites Al-Fatiha twice, he will have to perform sajdah sahv.
If a person begins to recite durood after tashahhud in first sitting and has recited as much as "Allahuma salli ala Muhammadin " he will have to carry out sajdah sahv.
If a masbuq commits mistake while he is completing his prayer independently, he has to perform sajdah sahv in the final sitting.
If a person develops a doubt during his prayer as to whether he has performed three rak'ahs or four and he does not have such a doubt normally, he should repeat the whole prayer. But if he generally has such doubts he should take into account the lesser number of rak'ahs (3) and complete his prayer with sajdah sahv.
If a person omits any sunnah or mustahabb parts in the prayer he need not perform sajdah sahv.
If a person has committed a mistake entailing sajdah sahv but completes his prayer without carrying out sajdah sahv and realizes this before the salutation he should immediately perform sajdah sahv and repeat tashahhud, durood, and complete his prayer provided that he has not turned his face away from the Qiblah nor has had any conversation with another person.
If a person performs only one sajdah in a rak'ah he may make it up in a subsequent rak'ah before tashahhud in the final sitting and perform sajdah sahv. If, however, he, remembers the mistake after tashahhud, he should perform the missed sajdah immediately and recite tashahhud, durood, etc. and complete the prayer with sajdah sahv as usual.
If a person has offered four rak'ahs instead of two rak'ahs during a journey when it is essential to shorten one's prayer, and has observed the first sitting duly, he should carry out sajdah sahv in the final sitting. In this case, his first two rak'ahs will be regarded as fard and the remaining two as nafl.