وَيَقُولُهَا الْمُؤْتَمُّ
Meaning: And the follower also says it.
The follower says Ameen when the imam has finished al-Fatihah.
إِذَا أَمَّنَ الْإِمَامُ فَأَمِّنُوا
Meaning: When the imam says Ameen, you too should say Ameen.
This hadith establishes the act in congregational prayer.