He prays with people who do not say Aameen out loud after al-Faatihah
Question

                Most of the mosques in our area do not think that we should say Aameen after al-Faatihah. I went to one of the imams and asked him, and he said that saying Aameen out loud is a weak (da‘eef) view, and it is better to say it quietly. Is this correct?
Praise be to Allah.Firstly:
What is indicated by the hadiths is that it is Sunnah for both the imam and the one who is praying behind him to say Aameen out loud in a prayer in which the recitation is done out loud.
Al-Bukhaari (780) and Muslim (410) narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “When the imam says ‘Aameen’ then say ‘Aameen,’ for if a person’s saying Aameen coincides with that of the angels, his previous sins will be forgiven.”  Ibn Shihaab said: The Messenger of Allah (blessings and peace of Allah be upon him) used to say, ‘Ameen.’.
Al-Bukhaari (782) narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “When the imam says ‘Ghayr il-maghdoobi ‘alayhim wa laa’d-daalleen (not of those who have evoked [Your] anger or of those who are astray) [al-Faatihah 1:7], then say ‘Aameen’, for if his saying that coincides with that of the angels, his previous sins will be forgiven.”
Abu Daawood (932) narrated that Waa’il ibn Hujr (may Allah be pleased with him) said: When the Messenger of Allah (blessings and peace of Allah be upon him) recited “wa laa’d-daalleen (or of those who are astray)” [al-Faatihah 1:7], he would say “Aameen,” raising his voice. This report was classed as saheeh by Shaykh al-Albaani in Saheeh Sunan Abi Daawood.
Ibn Qudaamah (may Allah be pleased with him) said in al-Mughni (1/291):
“When he said ‘wa laa’d-daalleen (or of those who are astray’ [al-Faatihah 1:7], he would say ‘Aameen’”: To sum up, saying Aameen after finishing al-Faatihah is Sunnah both for the imam and the one who is praying behind him… It is Sunnah for both the imam and the one who is praying behind him to say it out loud, in prayers in which recitation is done out loud, and to do it quietly in prayers in which recitation is done quietly.
Abu Haneefah, and Maalik according to one of the two views narrated from him, said: It is Sunnah to say it quietly, because it is a supplication (du‘aa’), so it is mustahabb (recommended) to say it quietly, like the tashahhud.
But in response to that we may note that the Prophet (blessings and peace of Allah be upon him) would say “Aameen,” raising his voice, and the Prophet (blessings and peace of Allah be upon him) enjoined saying Aameen when the imam says Aameen. If the imam does not say Aameen out loud, then no ruling can be attached to his Aameen, and it will be like the case when prayer is recited quietly. The saying of Aameen is connected to the recitation, and should be said out loud when the recitation is done out loud. End quote.
Secondly:
If a person is praying with people who do not think that it is Sunnah to say Aameen out loud, and his saying it out loud would cause trouble, then he should not say Aameen out loud in this instance. His not saying Aameen is refraining from doing a Sunnah action, but Islamic teaching and wisdom indicate that a person should not do Sunnah actions of that will lead to disputes and discord.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked:
Is it permissible not to say Aameen out loud and not to raise the hands?
He (may Allah have mercy on him) said: Yes, if he is among the people who do not raise their hands and do not say Aameen out loud. In that case it is better not to do that, so as to soften their hearts, and so that he will be able to call them to goodness, and to teach them and guide them, and so that it will be possible to reconcile between them. If he differs from them, they will be put off by that, because they think that this is the teaching of the religion; they think that one should not raise their hands except at the takbeerat al-ihraam [the takbeer at the beginning of the prayer]. They think that this is the teaching of the religion, and they do that with their scholars [?].
The same applies to not saying Aameen out loud, which is a well-known difference of opinion among the scholars; some of them say that it should be said out loud, and some say that it should not be said out loud. In the hadiths it says that the Prophet (blessings and peace of Allah be upon him) raised his voice when saying that, and in other hadiths it says that he kept his voice low.
Even though the correct view is that it is mustahabb to say Aameen out loud, and that it is something that is recommended, not doing something may be recommended [?]. So a believer may refrain from doing something that is recommended if it will lead to division, disputes and trouble. In fact [in that case] the believer should refrain from doing the thing that is recommended. With regard to one who seeks to call people to Allah, may He be glorified and exalted, if his refraining from doing it will serve the greater purpose SC then he should refrain from doing it. [?] An example of that is when the Prophet (blessings and peace of Allah be upon him) chose not to demolish the Ka ‘bah and rebuild it on the foundations of Ibraaheem; he said: “…because Quraysh have only recently left disbelief behind.” Hence he left it as it was, and did not change it, because that served the public interest.
End quote from Majmoo ‘ Fataawa Ibn Baaz (29/275).
And Allah knows best.