1 Samuel 6:9-10

9Then watch—if it goes up by the way of its own territory to Beth-shemesh, then it was He who inflicted on us this great harm. But if not, then we will know that it was not His hand that struck us; it just happened to us by chance.”

10So the people did so—they took two milk cows and hitched them to the cart, and shut up their calves at home.