The World Federation One Stop Fiqh

Ruling 981

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