Jeremiah 40:16

But Gedaliah the son of Ahikam said unto Johanan the son of Kareah, Thou shalt not do this thing: for thou speakest falsely of Ishmael.

Cross-reference

Jeremiah 41:2 records the assassination Gedaliah dismissed as false, showing his fatal error in rejecting Johanan's warning.

2 Kings 25:25 gives the same account of Ishmael assassinating Gedaliah, confirming the event from a parallel historical record.