It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "I prayed with the Prophet (peace and blessings be upon him). Whenever he (peace and blessings be upon him) recited 'Not of those who have earned [Your] anger, nor of those who are astray', he would say 'Amin', so that the believers would coincide with the angels."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18259