All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The Prophet, sallallaahu ‘alayhi wa sallam, said: “Verily, prayer occupies one’s full attention.” Hence, the worshipper should observe Khushoo‘ (submission and humility) while performing the prayer and should not make sounds or movements that contradict or undermine the due tranquility and humbleness in the prayer. In fact, producing such sounds does not befit the status of the prayer, whose essence is showing humility, tranquility, submission, and devotion before Allaah, The Exalted.
If the worshipper utters two letters while making such sounds, his prayer is considered invalid according to many of the jurists because this constitutes talking during the prayer. Verily, talking deliberately during the prayer invalidates it. Therefore, the worshipper should perform his prayer with full devotion and should not say anything other than the verses and supplications recited in the prayer.
Allaah Knows best.