Jump/leap to conclusions
Definition 1 of 1
Pro Tip 1/3Metaphorical Leap
Understand this phrase is a metaphor suggesting a person skips steps in a logical process. 
He saw her talking to his friend and leaped to conclusions about them dating.
Pro Tip 2/3Negative Connotation
Usually implies a hasty and negative judgment without sufficient evidence. 
She jumped to the conclusion that he stole the money, but it was actually a gift.
Pro Tip 3/3Cautionary Phrase
Often used to advise someone to wait for more information before deciding. 
Before you jump to conclusions, let's wait for the test results.