What is the ruling for one who forgets to say "Amen" after reciting Al-Fatiha in prayer, whether praying individually or in congregation?

Saying "Ameen" after Al-Fatiha in prayer is a recommended act (mustahabb). Forgetting to say it does not necessitate the prostration of forgetfulness (sujud al-sahw) nor does it affect the validity of the prayer. In fact, whoever performs the prostration of forgetfulness for intentionally or unintentionally omitting a recommended act, their prayer becomes invalid.