It is necessary for the follower that when he hears the Imam reciting (wa la ad-dallin) at the end of Surah Al-Fatihah, he should say Ameen—because the Noble Prophet صلی اللہ علیہ وسلم has commanded the follower to say Ameen when his Imam recites (wa la ad-dallin), just as the Prophet صلی اللہ علیہ وسلم has commanded the follower to say Ameen when the Imam says Ameen.

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When the Imam recites «‏‏‏‏غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ» ‏‏‏‏, you should say Ameen. Indeed, the angels also say Ameen and the Imam also says Ameen. So, whoever's Ameen coincides with the Ameen of the angels, his previous sins are forgiven." This is the narration of San‘ani.