Prayer: Remembering One or More Omitted Rakʿahs After Salaam
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1255
If after the salām of the prayer one remembers that he has not performed one rakʿah or more from the end of the prayer, in the event that he has done something that were he to do it intentionally or inadvertently during prayers it would invalidate the prayer – such as turning his back to qibla – his prayer is invalid. However, if he has not done anything that were he to do it intentionally or inadvertently during prayers it would invalidate the prayer, he must immediately perform what he had forgotten, and for the additional salām, he must, based on obligatory precaution, perform sajdatā al‑sahw.