Ameen at the end of the prayer. Imam and follower

The statement "aameen", is a sunnah for the imaam and for the follower. It is a sunnah for the imaam in the audible prayer; he raises his voice with it after the recitation of al-Faatihah, saying aameen. And it is a sunnah for the follower; he raises his voice with it once the Imaam reaches his saying: 

غَيْرِ آلْمَغْضُوبِ عَلَيْهِم وَ لا آلضَّآلِّين

Not the path of those who have earned your anger, nor those who have gone astray
[Sooratul-Faatihah, 1:7]

He is to say: Aameen.

The scholars, may Allaah have mercy upon them, have mentioned that this place is from the places wherein the Ta’meen (saying aameen) of the Imaam coincides with the Ta’meen of the follower. I asked Shaykh Naasirud-Deen al-Albaanee, may Allaah have mercy upon him, during his visit to the Kingdom of Saudi Arabia in the year 1406h., about the issue of the Ta’meen of the follower with the Ta’meen of the imaam; the shaykh, may Allaah have mercy upon him, said: “It is upon the follower to delay his ta’meen behind the ta’meen of the Imaam by the extent of a letter or two letters so that the ta’meen of the follower follows the ta’meen of the imaam and coincides with it in some of the letters. So when the Imaam says: ‘aameen,’ the follower begins the saying of: ‘aameen;’ therefore he and the imaam coincide in some of the letters of the word. The imaam is to precede him by a letter or two letters from the word. In doing so, agreement between the Imaam and follower in the saying of aameen is attained in some of the letters from the word; and by that the ta’meen of the follower occurring immediately after the Ta’meen of the Imaam, is attained.”1 

Footnotes:
1. Taken from Explanation of the Prophet's Prayer Described
Translated by: Raha ibn Donald Batts