Is it obligatory for the congregant to say "Ameen" immediately after finishing the recitation of Al-Fatiha, or to wait for the Imam to say "Ameen" with him?

The worshipper (ma'mum) must recite Surat Al-Fatihah in both silent and audible prayers. There is no specific time for its recitation; it can be recited when the Imam is silent, or during his recitation, or before or after him, depending on the worshipper's opportunity. If the worshipper finishes Al-Fatihah before the Imam, it is recommended for him to say "Ameen," then say "Ameen" again with the Imam, due to the prophetic command regarding that. If the worshipper and the Imam finish Al-Fatihah at the same time, then one "Ameen" is sufficient.