What kind of reading material are you looking for? Formally proven Smart Contracts would be the same as formally proven non-smart contract computer programs.

There is a paper[1] written by some researchers on how using a more powerful language (Such as Idris) could prevent a whole category of errors in smart contracts development, but it doesn't necessary talk about formal verification of smart contracts.

1. https://publications.lib.chalmers.se/records/fulltext/234939...