"The burden of proof is always on the accuser." Can you please give me an official reference for that? I know it is kind of off-topic and it should be obvious

"The burden of proof is always on the accuser." Can you please give me an official reference for that? I know it is kind of off-topic and it should be obvious as a common sense, but I am not joking here. When I asked a USA touristic visa I was told we are all considered by default guilty of illegal immigration until proven otherwise.