Prayer: Remembering a Forgotten Sajdah from the Last Rakʿah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1242
If a person remembers after the salām of the prayer that he has forgotten one sajdah from the last rakʿah, in the event that he does not do anything that invalidates prayers, he must perform it and all that follows it, i.e. tashahhud and salām. And based on obligatory precaution, he must perform sajdatā al‑sahw for saying an additional salām.