It was narrated from Abū Hurayrah (may Allah be pleased with him) that the the Prophet (may the peace and blessings of Allah be upon him) said:
"When the Imām says Aameen, then say Aameen, for the angels say Aameen, and if a person’s Aameen coincides with the Aameen of the angels, his previous sins will be forgiven." [al-Bukhāri: 780]
The companion Wā'il ibn Ḥujr (may Allah be pleased with him) said:
"When the Prophet (may the peace and blessings of Allah be upon him) recited the verse, 'Nor of those who go astray’ [Sūrah al-Fātiḥah, verse 7], he would say Aameen; raising his voice.” [Abū Dāwood: 932]