Prayer: Doubting Whether a Forgotten Sajdah Was Made Up

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 1248

If after prayers one doubts whether or not he has made up a forgotten sajdah, in the event that the time for the prayer has not expired, he must make up the sajdah. In fact, even if the time has expired, he must, based on obligatory precaution, make it up.

-Ayatullah Sistani, Practical Laws of Islam, Prayer (Salah)