"Amen" is a Hebrew word. At the time of Jesus (and before), it was often used at the end of a prayer to express one's agreement and support of what was said.
To use it as Jesus did, at the beginning of a sentence ("Amen, amen, I say to you ...), was unusual. It is not found on the lips of anyone in the New Testament except Jesus. Nor is it used by any of the early Church authors.
Scholars think that it goes back to the historical Jesus and is one of those times when we have not simply the substance of what Jesus said, but his own words.
Some translations of the gospels, in an effort to convey the sense of the "Amens," substitute phrases such as "Believe me," or "Truly," or "Very truly," or "In truth."
- Little White Book, Diocese of Saginaw