From Longman Dictionary of Contemporary Englishrest assured (that)rest assured (that)formalWORRIED used to tell someone not to worry, because what you say about a situation is true You may rest assured that it will be ready on time. → rest
Examples from the Corpus
rest assured (that)• Whoever wins the title, rest assured of one thing: The national champion would lose the next day.• Lasorda can rest assured that his place in baseball history is secure.• This may sound complicated, but rest assured, that it is simplicity itself.• Please rest assured that nothing will get beyond the earliest stages of preparation without full consultation taking place.• I can tell my grandsons they can rest assured that this is not Bosnia.• And of course you can rest assured the product contains no artificial colours, flavours or preservatives.• Think more positively and rest assured, the sky will be doing the same, too!• And rest assured there will be times when there is simply no humor to be found.• If that sounds at all complicated, rest assured you do get full instructions and diagrams. 