Islamic Laws; Ayatullah al Uzama Syed Ali al-Husaini Seestani
1001. It is Wajib for a man to recite Surah al-Hamd and the other Surah loudly, while offering Fajr, Maghrib and Isha prayers, and it is Wajib for a man and a woman to recite Surah al-Hamd and the other Surah silently while offering Zuhr and Asr prayers.
1002. As a precaution, men must take care to recite loudly every word of Surah al-Hamd and the other Surah, including their last letters, in the prayers of Fajr, Maghrib and Isha.
1004. If a person intentionally prays loudly where he should pray silently, and vice versa, his prayer is void. But, if, he does so owing to forgetfulness, or not knowing the rule, his prayer is in order. And if he realises that he is doing a mistake while reciting the Surah al-Hamd and the other Surah, it is not necessary to recite again what he has recited not following the rule.