The Bodhisatta was once a royal treasurer. While one of his slaves pounded rice, her mother was getting bitten by flies and asked her daughter to shoo them away. Without thinking, the daughter tried to kill the flies with her pestle and struck her mother so hard she died. When the Bodhisatta heard what happened, he told those around him that smart enemies are better than stupid friends.
In the Lifetime of the Buddha
While visiting the Buddha, the king’s treasurer told him one of his slaves had accidentally killed her mother by smashing her with a pestle while intending to kill flies. The Buddha told him this story so he knew that his slave and her mother had done the exact same thing in earlier births.