What is the ruling on saying "Subhan Allah" (Glory be to Allah) and the Imam abandoning the prostration of forgetfulness if he forgot Al-Fatiha in the third Rak'ah and did not prostrate for forgetfulness?

The Sunnah (definition) is to recite silently in the last two rak'ahs (units) of Dhuhr, Asr, and Isha prayers, and in the last rak'ah of Maghrib prayer. Reciting aloud in these rak'ahs does not invalidate the prayer, although it is better to recite silently, following the Sunnah. If the Imam recites aloud and is reminded by the congregants, that is good so he may follow the Sunnah, and he is not required to perform sujood as-sahw (prostration of forgetfulness), because both reciting aloud and silently are Sunnah acts, and sujood as-sahw is only required for acts that would otherwise invalidate the prayer.