Prayer: Forgetting a Sajdah or Tashahhud and Remembering Before Ruku
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1230
If a person forgets one sajdah or tashahhud and remembers it before performing rukūʿ of the next rakʿah, he must go back and perform it; and based on recommended precaution, he should perform sajdatā al‑sahw for the additional standing.