Numbers 32:17

32:17 but we will maintain ourselves in armed readiness and go before the Israelites until whenever we have brought them to their place. Our descendants will be living in fortified towns as a protection against the inhabitants of the land.

Numbers 35:15

35:15 These six towns will be places of refuge for the Israelites, and for the foreigner, and for the settler among them, so that anyone who kills any person accidentally may flee there.