What is entailed for someone who forgot to say "Amen" after Al-Fatiha or the Basmala before reciting the Qur'an in prayer?

Reciting the Basmalah (Bismillah Ar-Rahman Ar-Rahim) when reading Al-Fatihah or a Surah, and saying "Ameen" in prayer are among the Sunnah acts of prayer. If one forgets any of these, prostration of forgetfulness (sujud as-sahw) is not obligatory, though there is no harm in performing it, and the prayer remains valid. However, if a worshipper habitually performs a recommended Sunnah act and forgets it, it is legislated for him to perform the prostration of forgetfulness.