The World Federation One Stop Fiqh

Ruling 981

If when one must recite aloud he intentionally recites in a whisper, or, if when one must recite in a whisper he intentionally recites aloud, then based on obligatory precaution his prayer is invalid. However, if he does this due to forgetfulness or not knowing the ruling, his prayer is valid. If while reciting Sūrat al-Ḥamd or the other surah he realises that he has made a mistake, it is not necessary for him to repeat what he has already recited.