Prayer: Omitting or Adding a Sajdah During Sajdata al-Sahw
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1235
If a person knows that he has not performed one of the sajdata al‑sahw, and it is not possible to perform the other one on account of a long time having elapsed, or he knows that he inadvertently performed three sajdahs, then he must perform sajdatā al‑sahw.