The World Federation One Stop Fiqh

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.