"Rest assured" Vs " Be rest assured"

"Rest assured" Vs " Be rest assured"

Simply put, "Rest Assured" is the correct thing to say.

You can say either "Rest assured" or "Be assured"

These are imperative verbs ( the same as the base or infinite form)

You can not say " Be rest assured" because then you are using two imperative verbs in succesion, which is ungrammatical.

"Rest assured"  > Meaning:
To be assured, to be cirtain that something will happen.

Examples:
(1)  please rest assured that we will do everything possible to help.
(2)  I know this fellow well, and you can rest assured he will give you good advice.
(3)  You can rest assured that your account will be credited tomorrow.



Sources: Oxford English Dictionary, Collins English Dictionary.