Quite often, however, I have seen it used with a be before it, like in the following examples:
- It may sound quite amazing and perhaps unbelievable, but you can be rest assured that what you're about to read is completely true.
- By choosing us you can be rest assured that your facility, project...
- ...you can be rest assured you have full rights to use all images...
This is not correct.
The rest in rest assured is a verb which means to relax, to not worry. You'll understand the expression better if you imagine a comma after rest. So you could paraphrase the first example as
...but you can rest, assured in the knowledge that what you're about to read is completely true.Now you can easily see why be rest assured doesn't make sense.