Ruling 1224
It is not obligatory for one to perform sajdatā al‑sahw for the sound he makes when coughing; however, if one inadvertently groans, sighs, or says ‘oh’, then based on obligatory precaution, he must perform sajdatā al‑sahw.
Prayer: Coughing, Groaning, or Sighing During Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
