Deut 19:5

וַאֲשֶׁר֩ יָבֹ֨א אֶת־רֵעֵ֥הוּ בַיַּעַר֮ לַחְטֹ֣ב עֵצִים֒ וְנִדְּחָ֨ה יָד֤וֹ בַגַּרְזֶן֙ לִכְרֹ֣ת הָעֵ֔ץ וְנָשַׁ֤ל הַבַּרְזֶל֙ מִן־הָעֵ֔ץ וּמָצָ֥א אֶת־רֵעֵ֖הוּ וָמֵ֑ת ה֗וּא יָנ֛וּס אֶל־אַחַ֥ת הֶעָרִים־הָאֵ֖לֶּה וָחָֽי׃

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.