Jeremiah 42:5 in the King James Version
Then they said to Jeremiah, The LORD be a true and faithful witness between us, if we do not even according to all things for the which the LORD thy God shall send thee to us.
Context
4 Then Jeremiah the prophet said to them, “I have heard you. Behold, I will pray to the LORD your God according to your words; and it will happen that whatever thing the LORD answers you, I will declare it to you. I will keep nothing back from you.”
5 Then they said to Jeremiah, “May the LORD be a true and faithful witness among us, if we don’t do according to all the word with which the LORD your God sends you to tell us.
6 Whether it is good, or whether it is bad, we will obey the voice of the LORD our God, to whom we send you; that it may be well with us, when we obey the voice of the LORD our God.”