The scrum alliance / scrum foundation documents are not absolutely definitive on this however this is because it is implied all over the place. Logically the sprint backlog has to be ordered.
It is a more detailed sub set of the product backlog which is ordered and you plan by working through the product backlog in order of priority. Why wouldn't you work on the highest priority?
Minimizing work in progress and swarming stories is encouraged because you want to maximize the number of complete stories, not have several half finished ones at the end of the sprint as those don't meet the definition of done or have any business value.
Given the ethos of Scrum (prioritization, maximising business value, increasingly x-functional teams) it just wouldn't make sense for this to be unordered.