Rest assured

According to Cambridge Dictionary, rest assured means to be certain that something will happen and gives this example: You can rest assured that you're going to get a good deal.

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. 

Be rest assured

The expression rest assured should not be preceded by be like in the following examples:

• The investor community can be rest assured that the new government would not pursue any policy that will create fundamental difficulties for growth in savings and investment. (Indian Express)
• Twi-hard fans can be rest assured all is well in their world again. (okmagazine.com)
• Just like all Little Green Sheep mattresses they are 100% chemical free so parents can be rest assured their child is sleeping on a bed of the highest quality despite being away from home. (babyworld.co.uk)
• If you are unsure or not net savvy, please be rest assured, our technical team can do all that for you. (wpwebhost.com)

Rest assured, I checked it online.