Ameen..

As-Salaam alaikum,
  We traditionally say ameen, when the last ayat/verse of Suratul al-Fatiha is recited: 'GAYRUL MAGHRUBI ALAIHIM WALA-RAALEEN [Nor the path of those who go astray] during the canonical prayer [Sallat], or as part of other prayers or invocations when one person make a Du'a (supplication) on behalf of and in the presence of others; it is likewise customary for the congregation to say ameen (Amin).

As noted by Imam Sahl Ibn Abdullahi al-Tustari, may Allah be Merciful to him, in his Tafsir al-Tustari, Muhammad Ibn Sawwar related from Ibn uyayna from 'Amr Ibn Dinar on the authority of Jabir Ibn Abdullah, may Allah be pleased with him, who related that the Messenger of Allah, Sallallahu alaihi Wasallam, said:--

     ''Keep to the straight path [though] you will not be able to encompass
     [all good actions]. Know that the best of your actions is Prayer, and [indeed]
    only a believer observes ablution carefully. Whenever the Imam say Nor [the path]
    of those who go astray [1:7], you should say Ameen, for Allah is pleased with
    the one who says it, and He accepts his prayer and responds to his supplication.''

In another wording, on the authority of Abu Hurayra, may Allah be pleased with him, Al-Zuhri related from Ibn al-Musayyab that the Prophet, Sallallahu alaihi Wasallam, said:--

   "When the Imam says, Nor [the path] of those who go astray, say Ameen,
   for truly the angels say Ameen, and the one whose pronouncement of 'Ameen'
   is simultaneous with that of the angels will be forgiven for all his previous sins.''