(174) باب التَّأْمِينِ وَرَاءَ الإِمَامِ
Narrated Wa'il ibn Hujr:
When the Messenger of Allah (ﷺ) recited the verse "Nor of those who go astray" (Surah al-Fatihah, verse 7), he would say Amin; and raised his voice (while uttering this word).
Reference: Sunan Abi Dawud 932
In-book reference: Book 2, Hadith 543
English translation: Book 3, Hadith 932