Well, if God tell you so, you shouldn't eat that sandwich.
If you are familiar with chain of trust in computer security, Muslims have something similar for the Quran, there are hundred or thousands of peoples alive today who will give the names of their Quran teachers until your reach the Prophet, this will give you thousands of chains, with the exact Quran script, all people on this "chains" memories the Quran letter by letter.
In that script God tell Muslim to fast as Christian an Jewish did before them.