Musnad Abu Ya'la al-Mawsili 160:16

Abu Khaythama narrated to us, Jarir narrated to us, from Layth, from Ka'b, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Imam says: 'Not of those who have incurred [Your] anger, nor of those who are astray' (Al-Fatihah), and those behind him say: 'Amin,' and the 'Amin' of the people of the heaven meets the 'Amin' of the people of the earth, Allah forgives the servant what preceded of his sins." He said: "And the likeness of the one who does not say 'Amin' is like the likeness of a man who went on a military expedition with a people, and they drew lots, and their lots came out, but his lot did not come out. He said: 'Why did my lot not come out?' They said: 'Because you did not say Amin.'"