This action is a sunnah.
It means you can chose not to do it. but we want to chase for lots of ajr and does we do the as much sunnah as we can in our solah.
It is mustahab (recommended) to say Ameen after finsihing Al fateehah.
It means, “Oh Allah accept our invocation.”
And that is why when we hear someone makes dua’ we say Ameen. For Ameen is also a form of dua.
Coinciding with the angels
Prophetic Commentary on the Qur’an (Tafseer of the Prophet (pbuh))
Bukhari :: Book 6 :: Volume 60 :: Hadith 2
Narrated Abu Huraira:
Allah’s Apostle said, “When the Imam says: ‘Ghair-il-Maghdubi ‘Alaihim Walad-Dallin (i.e. not the path of those who earn Your Anger, nor the path of those who went astray (1.7)), then you must say, ‘Ameen’, for if one’s utterance of ‘Ameen’ coincides with that of the angels, then his past sins will be forgiven.”
We know the great benefit to say Ameen at the right time. Not before the imam, but together with the imam.
Imam Abu bakr says, ‘this hadith proves that the Imam must say Ameen loudly because the Prophet sallallahu alaihi wasalam would only order the folllowers to say Ameen with the imam if he knew that the imam was saying Ameen. No scholar can imagine that Rasulullah sallallahu alaihi wasalam would order a follower to say Ameen when he could not hear Ameen from the imam.
1. Don’t say Ameen before the imam
2. If you say ameen at the apropriate time, you will Insya Allah coincide with the angels. May Allah forgive our past sins.
3.If you are the imam, then do say Ameen loud and clear and the right timing too.