Normally found at the beginning of a sentence and used in every New Testament book but Acts, James, and Third John, the Greek interjection “amen” was a way of solemnly affirming that something was true.  Jesus first used this word in His Sermon on the Mount (Mt. 5:18, 26; 6:2, 5, 13, 16Mt. 5:18, 26; 6:2, 5, 13, 16
English: American Standard Version (1901) - ASV
18 For verily I say unto you, Till heaven and earth pass away, one jot or one tittle shall in no wise pass away from the law, till all things be accomplished. ²⁶ Verily I say unto thee, thou shalt by no means come out thence, till thou have paid the last farthing. ² When therefore thou doest alms, sound not a trumpet before thee, as the hypocrites do in the synagogues and in the streets, that they may have glory of men. Verily I say unto you, They have received their reward. ⁵ And when ye pray, ye shall not be as the hypocrites: for they love to stand and pray in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have received their reward. ¹³ And bring us not into temptation, but deliver us from the evil one. ¹⁶ Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may be seen of men to fast. Verily I say unto you, They have received their reward.

WP-Bible plugin), and He continued to use it throughout His ministry.  He even used it shortly before going to the cross (Jn. 16:23Jn. 16:23
English: American Standard Version (1901) - ASV
23 And in that day ye shall ask me no question. Verily, verily, I say unto you, if ye shall ask anything of the Father, he will give it you in my name.

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Paul’s use of this word in 1 Cor. 14:161 Cor. 14:16
English: American Standard Version (1901) - ASV
16 Else if thou bless with the spirit, how shall he that filleth the place of the unlearned say the Amen at thy giving of thanks, seeing he knoweth not what thou sayest?

WP-Bible plugin indicates “amen” was also used at the end of prayers.

In the ASV and KJV “amen” is often translated “verily.”