Musannaf Ibn Abi Shayba 3:5587

Abu Khalid al-Ahmar narrated to us, from Ibn ‘Ajlan, from Zayd ibn Aslam, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Imam was only appointed to be followed. So when he says, 'Not of those who have evoked [Your] anger or of those who are astray,' then say, 'Amin.'"