If One Realises After Salām That He Has Not Offered One Rak‘at or More
Issue No.1117- If a person realises before the salām of prayer that he has not offered one rak‘at or more of the final rak‘ats of prayer, he should perform the rak‘at which had been forgotten, and his prayer is correct. However, if he realises this after the salām of prayer, and he has done an act which doing it intentionally or inadvertently invalidates the prayer (e.g. turning one's back towards the qiblah), the prayer will be void. In case however, he has not performed such an act, he should immediately perform the forgotten rak‘ats and the prayer will be valid, and he should, as an obligatory precaution, perform sajdah al-sahw for the undue salām.
