These words are occasionally used as synonyms, but they do have distinct meanings. Evidence is information given in an investigation to support a contention: "At the trial, the witness presented evidence to convict the accused." Proof is that kind of evidence which is so weighty as to remove any possible doubt: "His signed confession is proof of his guilt." Testimony is the statement of a witness, usually given under oath: "The jury listened attentively to the testimony." Evidence and testimony are often confused, but testimony should be used to refer only to statements and evidence to any ground for belief that is spoken, written, or presented in any other form.
Dictionary of problem words and expressions. Harry Shaw. 1975.