Sajdah Sahv (sajdah for forgetful mistakes) is performed (by the specified method) after a wajib salat when certain types of mistakes occur forgetfully during that salat. There are 5 types of mistakes, for which it is wajib to perform Sajdah Sahv. There are 3 types of mistakes, for which it is recommended (mustahab) to perform Sajdah Sahv.

Sajdah Sahv means performing two sajdas with zikr, reciting Tashahhud and Salam, soon after completing the salat, for each type of such forgetful mistake during the salat, as explained later. Thus if you make 3 mistakes in one salat, then three separate sets of Sajdah Sahv are to be performed one after another, after completing the salat.

(1) Talking forgetfully during salat (or even uttering a single word which has a meaning, e.g. saying ‘Ah’ in pain).

(2) Reciting Salam forgetfully at a wrong place during salat. (For example, if you recite Salam after Tashahhud in 2nd rakat in a 4-rakat salat). Salam in a salat has three parts. Sajdah Sahv becomes wajib if 2nd or 3rd part of Salam is recited forgetfully. If only 1st part is recited forgetfully, then Sajdah Sahv is not wajib, but it is mustahab to perform.

(3) Forgetting to recite Tashahhud during salat. If you forget Tashahhud after 2nd rakat, but remember it before the ruku of next rakat, then you should return and perform it. And after the salat, perform Sajdah Sahv for performing additional standing (qiyam).

(4) Adding or omitting something forgetfully during salat, but that addition or omission does not make salat void. (Examples:
(i) reciting Tashahhud after 1st rakat or after 3rd rakat in a 4-rakat salat;
(ii) doing an extra sajda by mistake;
(iii) forgetting zikr in a sajdah or ruku,
(iv) forgetting second surah in 1st rakat or 2nd rakat, etc).

(5) When doubts occurs in the number of rakats in a 4-rakat salat, if it is 4th or 5th rakat, or it is 4th or 6th rakat, or it is 4th or more rakat (as explained in Fiqh Notes #2). 
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(1) Forgetting one sajdah in a rakat. But after salat, you must perform qadha of that sajdah. If you forget one sajdah, and remember it before the ruku of the next rakat, you should return and perform it, and after the salat, perform Sajdah Sahv for additional standing (qiyam)

(2) Standing up forgetfully where you are supposed to sit, or keep sitting forgetfully where you are supposed to stand up during salat. For example, you start standing after 2nd rakat without reciting Tashahhud, or you keep sitting after 1st rakat or after 3rd rakat, in a 4rakat salat).

(3) Reciting 1st part of Salam forgetfully at a wrong place in a salat.

Immediately after completing the salat,

(1) Make a Niyyat of performing Sajdah Sahv for that mistake. No Takbeeratul Ihram.

(2) Do 1st Sajdah, and recite the zikr “Bismillahi wa billahi assalamu alayka ayyuhan nabiyyu wa rahmatullahi wa barakatuh”

(3) Sit down, and do 2nd Sajdah, and recite the above zikr

(4) Sit down, recite Tashahhud and Salam. (in Salam only Assalamu Alaikum is wajib but it is better to add warahmatullahi wa barakatuh).

(a) Sajdah Sahv must be performed immediately after finishing the salat. If it is delayed intentionally, then you are committing a sin, but must be performed at once.

(b) If you forgot to perform Sajdah Sahv altogether after the salat, then you should perform it immediately as soon as you remember it. It is not necessary to repeat your salat. (sentence corrected on 16.01.07)

(c) Sajdah Sahv must be performed on the object on which Sajdah is allowed.

(d) If you forgot one Sajdah and also Tashahhud in a salat, then after salat, you must first perform qadha of one Sajdah, then recite Tashahhud as qadha, then perform Sajdah Sahv for forgetting the Tashahhud, and then perform Sajda Sahv for forgetting one Sajdah as a precaution.
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