1 Chronicles 16:36. All the people said, Amen, and praised the Lord.
Amen literally means firm, from aman, to prop, to support. Its figurative meaning is faithful. Its use is designed as a confirmatory response, and the custom is very ancient. See Numbers 5:22; Deuteronomy 27:15-26.
“The Jewish doctors give three rules for pronouncing the word: 1. That it be not pronounced too hastily and swiftly, but with a grave and distinct voice. 2. That it be not louder than the tone of him that blessed. 3. It was to be expressed in faith, with a certain persuasion that God would bless them and hear their prayer” (Burder, Oriental Customs, No. 438).
It is also customary for the Mohammedans, at the close of every public prayer, to say, Amen.