What kind of evidence are you looking for? I can point you to public health authorities saying immunity works and lasts for ~a year [1][2], but I suspect you've seen these statements already. If you're looking for a scientific study unambiguously proving it, we of course won't be able to have that until a year has passed.

[1] https://www.businessinsider.com/coronavirus-fauci-those-who-...

[2] https://www.bbc.com/news/health-52446965