be rest assured

Rest assured means to be certain, to rest easy in the knowledge that, or to be consoled that the mentioned fact or circumstance is true. It has become such a popular idiom that many companies use it in their slogans such as Rest Assured System, and Rest Assured MC.

use it at the start of the sentence

e.g

"rest assured, my deed will be done"

It is incorrect because of redundancy. It is correct to use the words separately like "Please be assured..." or "Rest assured that..."

'Rest' can be a noun ('I need a rest') or a verb ('rest assured').