leap to conclusions
To make decisions or form opinions before one has all the pertinent facts. I know you found some suspicious things in her office, but don't leap to conclusions—talk to her first and see if she has a reasonable explanation. You're always leaping to conclusions whenever I make the slightest criticism of our relationship!
jump/leap to conˈclusions
make a decision about somebody/something too quickly, before you know or have thought about all the facts: There you go again — jumping to conclusions. Wait till you hear my side of the story!