Do you need to be able to *prove* a post said something, or do you want to *know* (for yourself) that a post said something? If the former, you would have to use something like archive.md or the wayback machine. If it is the later, you can write a script to scrape the person's post history and save it somewhere for you. To my knowledge, no archival services offer API access that can be used to save a page.