ἀμήνamḗn

properly, firm, i.e. (figuratively) trustworthy; adverbially, surely (often as interjection, so be it)

Greek (New Testament) · G281

Meanings

2. As

(a) in solemn assent to the statements or prayers of another (Numbers 5:22, Nehemiah 5:13, Nehemiah 8:6 etc.): ὁ ἀ., 1 Corinthians 14:16

(b) similarly, at the end of one's own prayer or ascription of praise: Romans 1:25, Romans 15:33, Galatians 1:5, 1 Timothy 1:17

(c) in the Gospels, exclusively, introducing solemn statements of our Lord, truly, verily: Matthew 5:18, Matthew 5:26, Mark 3:28, Luke 4:24, and elsewhere; ἀ. ἀ., always in John 1:52, John 3:3, John 5:19, and elsewhere; τὸ ναί, καὶ... τὸ ἀ., 2 Corinthians 1:20.

Appears 127 times in the Bible.

Matthew 5:18 I tell you the truth, until heaven and earth disappear, not even the smallest detail of God’s law will disappear until its purpose is achieved.
Matthew 5:26 And if that happens, you surely won’t be free again until you have paid the last penny.
Matthew 6:2 When you give to someone in need, don’t do as the hypocrites do—blowing trumpets in the synagogues and streets to call attention to their acts of charity! I tell you the truth, they have received all the reward they will ever get.
Matthew 6:5 “When you pray, don’t be like the hypocrites who love to pray publicly on street corners and in the synagogues where everyone can see them. I tell you the truth, that is all the reward they will ever get.
Matthew 6:16 “And when you fast, don’t make it obvious, as the hypocrites do, for they try to look miserable and disheveled so people will admire them for their fasting. I tell you the truth, that is the only reward they will ever get.
Matthew 8:10 When Jesus heard this, he was amazed. Turning to those who were following him, he said, “I tell you the truth, I haven’t seen faith like this in all Israel!

See every passage

Source: G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), public domain.