Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "When the imam recites 'not of those who earned anger, nor of those who went astray', and those behind him say 'Amin', and the 'Amin' of the people of the heavens (that is, the angels) and the people of the earth (that is, human beings) coincides, Allah Almighty will forgive the servant his past sins." He also said: "The example of the one who does not say 'Amin' is like a man who takes part in a military expedition with some people. They cast lots, and the lots of those people come out but his does not. He says: 'What is the reason that my share has not come out?' The other person says: 'Because you did not say Amin.'" (Allamah al-Haythami says:) I say: Part of it is reported in the Sahih.
Abu Ya'la recorded it; its chain contains Layth bin Abi Sulaym, who is thiqah (trustworthy) and a mudallis (one who conceals defects in the chain), and he has narrated it with 'an (mu'an'an).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2664