It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the imam says: 'Not the path of those who have earned wrath, nor of those who have gone astray' [Surah al-Fatihah: 7], then say: Amin, and Allah will answer your supplication."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6861