I say, and with God's success: The original rule for the last two rak'ahs is to recite Al-Fatiha specifically, and if one praises (God) or remains silent for the duration of it, it is valid. If one adds to the Surah Al-Fatiha, it does not require prostration for forgetfulness, and it is disliked as a matter of etiquette. And God knows best.