Deuteronomy 19:4-5

Now this is the situation regarding the manslayer who flees to one of these cities to save his life, having killed his neighbor accidentally, without intending to harm him:

If he goes into the forest with his neighbor to cut timber and swings his axe to chop down a tree, but the blade flies off the handle and strikes and kills his neighbor, he may flee to one of these cities to save his life.