(Imām Bukhārī narrates): 'Abdullāh ibn Yūsuf narrated to us, he said: Imām Mālik informed us, from Ibn Shihāb, from Sa'īd ibn al-Musayyab and Abī Salamah ibn 'Abdur-Rahmān, both of them informed him, from Abū Hurairah, may Allāh be pleased with him, he narrates that the Prophet ﷺ said:

When the imām says "Āmīn," you say "Āmīn," for indeed whoever's "Āmīn" coincides with the "Āmīn" of the angels, his past sins will be forgiven.

And Ibn Shihāb said: And the Messenger of Allāh ﷺ used to say "Āmīn." This is evident according to the heading, because the Messenger of Allāh ﷺ commanded the people that when the imām says "Āmīn," they should say "Āmīn."