הלכה אויף יונה 3:3

You find from what was learned that a person is permitted on the Sabbath to walk the entire city, even if it was [as big as] Nineveh, whether it was surrounded by a wall or not surrounded by a wall. And similarly, it is permitted to walk outside the city 2000 cubits in every direction, square, like a square tablet such that one benefits from the corners. And if one went outside 2000 cubits, we lash him with lashes of rebellion until 12 mil. But if he went out and distanced [himself] from the city more than 12 mil, even one cubit, he is lashed from the [law of] the Torah.