In an unusual move, FBI agents working the Hillary Clinton e-mail case had to sign a special form reminding them not to blab about the probe to anyone unless called to testify.

Sources said they had never heard of the “Case Briefing Acknowledgment” form being used before, although all agents must initially sign nondisclosure agreements to obtain security clearance.

“This is very, very unusual. I’ve never signed one, never circulated one to others,” said one retired FBI chief.

An FBI agent currently on the job admitted, “I have never heard of such a form. Sounds strange.”