Should the Imam and the follower say 'Amin' audibly or silently?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
It is recommended that the Imam and the follower recite 'Amin' audibly when the prayer recitation is audible, and silently when it is silent. This is supported by the narration that the Prophet said 'Amin' and raised his voice with it. Furthermore, if the follower were to say 'Amin' silently when the Imam said it audibly, there would be no basis for the command to align the 'Amin' utterances.
Supporting text
Abu Hanifa and one narration from Malik hold that 'Amin' should be recited silently because it is a supplication (Dua'), which is generally preferred to be said quietly, like the Tashahhud.