Authentic Supplication Number 81.

It is reported by way of ‘Abi Hurayrata  that the Messenger of Allah  said: “If the Imam says: Ghayril maghdhoobi `alayhim waladh-dhalleen then say ‘Ameen for the one whose saying coincides with the saying of the Angels, would have what preceded of his sins forgiven for him.

Reporters.
‘Al-‘Adawee said it was sound, and that it was reported by Al-Bukhari, page 159 of volume 8 of Fath AI-Baree; An-Nisa’ee, page 144 of volume 2; At-Tirmithi, page 78 of volume 2, who said it was comely-sound; and Ad-Darami (284/1).