rest assured

You can be sure, as in Rest assured that the police will recover your diamonds. This expression uses assured in the sense of “certain” or “confident,” a usage dating from the early 1500s.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.