This is one of the more straight forward questions and the response should be easy. I’ve checked this with my lawyer in the past and below is my suggestion.
1- The passport stamp (make sure that the officer stamps your entry - you might need to remind the officer - once you show the residency card they might simply check the residency and give back the passport to you)
2- Keep the boarding passes (in and out)
3- To be super safe, buy something everyday with your NIF so that you have a receipt with your NIF for those 14 days… It can be anything… Coffee, subway ticket, any small purchase that comes with a receipt with your NIF counts. (make sure the receipt is readable)
The above should be enough
- It only helps if you get a receipt from your hotel as well - my hotel once provided a written statement that I stayed there between X - Y dates (I don’t think this is really needed)
If you get a receipt with your NIF everyday that alone should be sufficient, yet boarding passes are equally important. They kind of close the loop, Mr E flew into Portugal on XYZ, here are the receipts with NIF numbers for that 2 weeks, then he flew out… As simple as that…