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).
Sunan Abi Dawud Book 2, Hadith 543
← Previous Hadith what do you mean by looking at me with furtive glances. Then they glorified Allah. When the prophet (ﷺ) finished his prayer, he asked; who was the speaker? The Prophet told him; this Bedouin. The Messenger of Allah (ﷺ) called me and said to me: Prayer is meant for the recitation of the Quran, and making mention of Allah. When you are in it (prayer), this should be your work therein. I never saw an instructor more lenient than the Messenger of Allah (ﷺ)

Sunan Abi Dawud Book 2, Hadith 542 Next Hadith → Wail b, hujr said that he prayed behind the Messenger of Allah (ﷺ),and he said Amin loudly and saluted at his right and left sides until I saw the whiteness of his cheek.

Sunan Abi Dawud Book 2, Hadith 544