The Bodhisatta was once a royal treasurer. While one of his slaves pounded rice, her mother came to visit. While the mother was lying down, she was being bit by flies and asked her daughter to shoo them away. Instead, without thinking, the daughter tried to kill the flies with her pestle, and hit her mother so hard that 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 slave girls had 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.