Is it permissible for the one being led in prayer (ma'mum) to say "Ameen" in the obligatory prayer when the Imam recites a surah that contains a supplication, such as the concluding verses of Surat Al-Baqarah?

What is required of the congregant is to listen attentively to the Imam's recitation and not to be preoccupied with supplication (du'a) or seeking forgiveness (istighfar). If he says "Ameen" when a supplication other than Al-Fatihah is recited, or if he asks (of Allah) when a verse of mercy is recited, or seeks refuge (in Allah) when a verse of punishment is recited, his prayer is not invalidated thereby. However, it is more befitting for him to listen attentively.