Charles Ellicott Commentary Jeremiah 42:5-6

"Then they said to Jeremiah, Jehovah be a true and faithful witness amongst us, if we do not according to all the word wherewith Jehovah thy God shall send thee to us. Whether it be good, or whether it be evil, we will obey the voice of Jehovah our God, to whom we send thee; that it may be well with us, when we obey the voice of Jehovah our God." — Jeremiah 42:5-6 (ASV)

The Lord be a true and faithful witness ... —The emphatic solemn oath implies that they are ready to accept the punishment that the righteous Judge will inflict if they prove unfaithful to their promise. The name of the place where they had recently been staying may have reminded them of the history of Genesis 31:49 (though that refers to another Mizpah), in which we find the same formula.