How can I provide proof that my paper has been proofread by a native speaker? I have submitted my manuscript to a journal. After several weeks the editor sent me the comments of two reviewers. The editor said that "the authors should provide proof that the paper has been proofread by a native English speaker".

So how can I provide the proof? Should I send the proofreading certificate which I recieved after using the service from a proofreading company?

And if I only ask a native speaker for proofreading without any certificates, how should i provide the proof?
This seems like an unreasonable request. Frankly, it does not matter if you paper has been proof-read by a native speaker or not. If, after revision, your English presentation is still bad, the editor won't accept it even if you include proof that it was read by a native speaker.

I would personally be prone to just ignore this request, make sure that the English language is as good as it can be, and resubmit the paper.