Why do we pray two rak'ahs aloud and two rak'ahs silently, and is there a religious text from the Prophet, peace and blessings be upon him, to that effect? And why does the worshiper recite Al-Fatiha only in the last two rak'ahs, and is there a religious text from the Prophet, peace and blessings be upon him, to that effect?

Praying two rak'ahs audibly, then two rak'ahs or one rak'ah silently in 'Isha and Maghrib prayers, is a confirmed Sunnah from the Prophet, peace and blessings be upon him. It is authentically reported from him that he would recite audibly in the first two rak'ahs of Maghrib and 'Isha, and silently in the last two. As for reciting only Al-Fatiha in the last two rak'ahs, this has been reported from the Prophet, peace and blessings be upon him, and it is the view held by the majority of scholars.