Short answer
If you remember before doing anything that ends the prayer, such as talking, turning away from the qibla or leaving your place, do the two sajdahs of sahw then, followed by the tashahhud and salam. If you have already done one of those things, sahw can no longer be done, and in the Hanafi school the prayer should be repeated within its time.
Sajdah sahw is wajib when a wajib act was missed or delayed by mistake. If you gave salam forgetting it, you may still do it as long as you are in your place facing the qibla and have not spoken or done anything that cuts the prayer. Once that chance has passed, the prayer is deficient: repeating it within its time is wajib, and after the time it is recommended.
Other madhhabs
- Shafi'i
- Sajdah sahw is sunnah. If you forget it and only a short time has passed, do it; if a long time has passed, leave it, and the prayer is valid.
- Hanbali
- Sajdah sahw is wajib where it is due. If you forget it, do it as long as not much time has passed; after a long gap or leaving the masjid it drops, and the prayer stays valid.
Explanation
Sajdah sahw (the prostration of forgetfulness) repairs a prayer in which a wajib act was missed by mistake. The Prophet ﷺ once finished a prayer and was told he had prayed more or less than usual; he turned to face the qibla, made two sajdahs and gave salam, and said he was human and forgot as people forget. So the sahw can be done after the salam when you remember straight away.
You remember while still sitting, after the salam: do it. Make two sajdahs with takbirs, sit, recite the tashahhud, salawat and du'a, and give salam. In the Hanafi method you would normally give one salam to the right before the sahw; if you already gave both, still do the sahw.
You remember after you spoke, stood up and left, or turned away: the chance for sahw has passed. Check whether the mistake actually needed sahw:
- It did not (for example you missed a sunnah, like the thana): nothing is needed.
- It did (for example you skipped the first tashahhud): repeat the prayer within its time. If the time has gone, repeating it is recommended, and do not worry over it.
- You missed a fard act (for example a whole sajdah or rakah): sahw would not have fixed it anyway; the prayer must be prayed again. See forgetting a rakah.
Behind an imam: you do not do sahw for your own mistakes; see does the follower do sajdah sahw. For the method in full, read sajdah sahw, and for what counts as wajib, the wajib acts of salah.
The evidence
Sahih Muslim 572
Sahih... إنما أنا بشر أنسى كما تنسون فإذا نسيت فذكروني ...
"I am only human; I forget as you forget, so when I forget, remind me. And when one of you is unsure in his prayer, let him aim for what is right, complete on that basis, then make two sajdahs."
Forgetfulness in prayer is repaired with two sajdahs, even after the salam.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Salah, chapter on sajdah sahw. Sahw is wajib; it remains possible after the salam unless something that cuts the prayer has occurred.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on sajdah sahw. A prayer with a wajib missed and not repaired is repeated within its time as a duty, and after it as a recommendation.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.